Set CEXTERNAL_FLAGS early on create()

[akaros.git] / kern / src / ns / qio.c

/* Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
 * Portions Copyright © 1997-1999 Vita Nuova Limited
 * Portions Copyright © 2000-2007 Vita Nuova Holdings Limited
 *                                (www.vitanuova.com)
 * Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
 *
 * Modified for the Akaros operating system:
 * Copyright (c) 2013-2014 The Regents of the University of California
 * Copyright (c) 2013-2015 Google Inc.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE. */

#include <vfs.h>
#include <kfs.h>
#include <slab.h>
#include <kmalloc.h>
#include <kref.h>
#include <string.h>
#include <stdio.h>
#include <assert.h>
#include <error.h>
#include <cpio.h>
#include <pmap.h>
#include <smp.h>
#include <ip.h>

#define PANIC_EXTRA(b)                                                  \
{                                                                       \
        if ((b)->extra_len) {                                           \
                printblock(b);                                          \
                backtrace();                                            \
                panic("%s doesn't handle extra_data", __FUNCTION__);    \
        }                                                               \
}

static uint32_t padblockcnt;
static uint32_t concatblockcnt;
static uint32_t pullupblockcnt;
static uint32_t copyblockcnt;
static uint32_t consumecnt;
static uint32_t producecnt;
static uint32_t qcopycnt;

static int debugging;

#define QDEBUG  if(0)

/*
 *  IO queues
 */

struct queue {
        spinlock_t lock;;

        struct block *bfirst;           /* buffer */
        struct block *blast;

        int len;                                        /* bytes allocated to queue */
        int dlen;                                       /* data bytes in queue */
        int limit;                                      /* max bytes in queue */
        int inilim;                             /* initial limit */
        int state;
        int eof;                                        /* number of eofs read by user */

        void (*kick) (void *);          /* restart output */
        void (*bypass) (void *, struct block *);        /* bypass queue altogether */
        void *arg;                                      /* argument to kick */

        struct rendez rr;                       /* process waiting to read */
        struct rendez wr;                       /* process waiting to write */
        qio_wake_cb_t wake_cb;          /* callbacks for qio wakeups */
        void *wake_data;

        char err[ERRMAX];
};

enum {
        Maxatomic = 64 * 1024,
        QIO_CAN_ERR_SLEEP = (1 << 0),   /* can throw errors or block/sleep */
        QIO_LIMIT = (1 << 1),                   /* respect q->limit */
        QIO_DROP_OVERFLOW = (1 << 2),   /* alternative to setting qdropoverflow */
        QIO_JUST_ONE_BLOCK = (1 << 3),  /* when qbreading, just get one block */
        QIO_NON_BLOCK = (1 << 4),               /* throw EAGAIN instead of blocking */
        QIO_DONT_KICK = (1 << 5),               /* don't kick when waking */
};

unsigned int qiomaxatomic = Maxatomic;

static size_t copy_to_block_body(struct block *to, void *from, size_t copy_amt);
static ssize_t __qbwrite(struct queue *q, struct block *b, int flags);
static struct block *__qbread(struct queue *q, size_t len, int qio_flags,
                              int mem_flags);
static bool qwait_and_ilock(struct queue *q, int qio_flags);

/* Helper: fires a wake callback, sending 'filter' */
static void qwake_cb(struct queue *q, int filter)
{
        if (q->wake_cb)
                q->wake_cb(q, q->wake_data, filter);
}

void ixsummary(void)
{
        debugging ^= 1;
        printd("pad %lu, concat %lu, pullup %lu, copy %lu\n",
                   padblockcnt, concatblockcnt, pullupblockcnt, copyblockcnt);
        printd("consume %lu, produce %lu, qcopy %lu\n",
                   consumecnt, producecnt, qcopycnt);
}

/*
 *  pad a block to the front (or the back if size is negative)
 */
struct block *padblock(struct block *bp, int size)
{
        int n;
        struct block *nbp;
        uint8_t bcksum = bp->flag & BCKSUM_FLAGS;
        uint16_t checksum_start = bp->checksum_start;
        uint16_t checksum_offset = bp->checksum_offset;
        uint16_t mss = bp->mss;

        QDEBUG checkb(bp, "padblock 1");
        if (size >= 0) {
                if (bp->rp - bp->base >= size) {
                        bp->checksum_start += size;
                        bp->rp -= size;
                        return bp;
                }

                PANIC_EXTRA(bp);
                if (bp->next)
                        panic("padblock %p", getcallerpc(&bp));
                n = BLEN(bp);
                padblockcnt++;
                nbp = block_alloc(size + n, MEM_WAIT);
                nbp->rp += size;
                nbp->wp = nbp->rp;
                memmove(nbp->wp, bp->rp, n);
                nbp->wp += n;
                freeb(bp);
                nbp->rp -= size;
        } else {
                size = -size;

                PANIC_EXTRA(bp);

                if (bp->next)
                        panic("padblock %p", getcallerpc(&bp));

                if (bp->lim - bp->wp >= size)
                        return bp;

                n = BLEN(bp);
                padblockcnt++;
                nbp = block_alloc(size + n, MEM_WAIT);
                memmove(nbp->wp, bp->rp, n);
                nbp->wp += n;
                freeb(bp);
        }
        if (bcksum) {
                nbp->flag |= bcksum;
                nbp->checksum_start = checksum_start;
                nbp->checksum_offset = checksum_offset;
                nbp->mss = mss;
        }
        QDEBUG checkb(nbp, "padblock 1");
        return nbp;
}

/*
 *  return count of bytes in a string of blocks
 */
int blocklen(struct block *bp)
{
        int len;

        len = 0;
        while (bp) {
                len += BLEN(bp);
                bp = bp->next;
        }
        return len;
}

/*
 * return count of space in blocks
 */
int blockalloclen(struct block *bp)
{
        int len;

        len = 0;
        while (bp) {
                len += BALLOC(bp);
                bp = bp->next;
        }
        return len;
}

/*
 *  copy the  string of blocks into
 *  a single block and free the string
 */
struct block *concatblock(struct block *bp)
{
        int len;
        struct block *nb, *f;

        if (bp->next == 0)
                return bp;

        /* probably use parts of qclone */
        PANIC_EXTRA(bp);
        nb = block_alloc(blocklen(bp), MEM_WAIT);
        for (f = bp; f; f = f->next) {
                len = BLEN(f);
                memmove(nb->wp, f->rp, len);
                nb->wp += len;
        }
        concatblockcnt += BLEN(nb);
        freeblist(bp);
        QDEBUG checkb(nb, "concatblock 1");
        return nb;
}

/* Makes an identical copy of the block, collapsing all the data into the block
 * body.  It does not point to the contents of the original, it is a copy
 * (unlike qclone).  Since we're copying, we might as well put the memory into
 * one contiguous chunk. */
struct block *copyblock(struct block *bp, int mem_flags)
{
        struct block *newb;
        struct extra_bdata *ebd;
        size_t amt;

        QDEBUG checkb(bp, "copyblock 0");
        newb = block_alloc(BLEN(bp), mem_flags);
        if (!newb)
                return 0;
        amt = copy_to_block_body(newb, bp->rp, BHLEN(bp));
        assert(amt == BHLEN(bp));
        for (int i = 0; i < bp->nr_extra_bufs; i++) {
                ebd = &bp->extra_data[i];
                if (!ebd->base || !ebd->len)
                        continue;
                amt = copy_to_block_body(newb, (void*)ebd->base + ebd->off, ebd->len);
                assert(amt == ebd->len);
        }
        /* TODO: any other flags that need copied over? */
        if (bp->flag & BCKSUM_FLAGS) {
                newb->flag |= (bp->flag & BCKSUM_FLAGS);
                newb->checksum_start = bp->checksum_start;
                newb->checksum_offset = bp->checksum_offset;
                newb->mss = bp->mss;
        }
        copyblockcnt++;
        QDEBUG checkb(newb, "copyblock 1");
        return newb;
}

/* Returns a block with the remaining contents of b all in the main body of the
 * returned block.  Replace old references to b with the returned value (which
 * may still be 'b', if no change was needed. */
struct block *linearizeblock(struct block *b)
{
        struct block *newb;

        if (!b->extra_len)
                return b;
        newb = copyblock(b, MEM_WAIT);
        freeb(b);
        return newb;
}

/* Make sure the first block has at least n bytes in its main body.  Pulls up
 * data from the *list* of blocks.  Returns 0 if there is not enough data in the
 * block list. */
struct block *pullupblock(struct block *bp, int n)
{
        int i, len, seglen;
        struct block *nbp;
        struct extra_bdata *ebd;

        /*
         *  this should almost always be true, it's
         *  just to avoid every caller checking.
         */
        if (BHLEN(bp) >= n)
                return bp;

        /* If there's no chance, just bail out now.  This might be slightly wasteful
         * if there's a long blist that does have enough data. */
        if (n > blocklen(bp))
                return 0;
        /* a start at explicit main-body / header management */
        if (bp->extra_len) {
                if (n > bp->lim - bp->rp) {
                        /* would need to realloc a new block and copy everything over. */
                        panic("can't pullup %d bytes, no place to put it: bp->lim %p, bp->rp %p, bp->lim-bp->rp %d\n",
                                        n, bp->lim, bp->rp, bp->lim-bp->rp);
                }
                len = n - BHLEN(bp);
                /* Would need to recursively call this, or otherwise pull from later
                 * blocks and put chunks of their data into the block we're building. */
                if (len > bp->extra_len)
                        panic("pullup more than extra (%d, %d, %d)\n",
                              n, BHLEN(bp), bp->extra_len);
                QDEBUG checkb(bp, "before pullup");
                for (int i = 0; (i < bp->nr_extra_bufs) && len; i++) {
                        ebd = &bp->extra_data[i];
                        if (!ebd->base || !ebd->len)
                                continue;
                        seglen = MIN(ebd->len, len);
                        memcpy(bp->wp, (void*)(ebd->base + ebd->off), seglen);
                        bp->wp += seglen;
                        len -= seglen;
                        ebd->len -= seglen;
                        ebd->off += seglen;
                        bp->extra_len -= seglen;
                        if (ebd->len == 0) {
                                kfree((void *)ebd->base);
                                ebd->off = 0;
                                ebd->base = 0;
                        }
                }
                /* maybe just call pullupblock recursively here */
                if (len)
                        panic("pullup %d bytes overdrawn\n", len);
                QDEBUG checkb(bp, "after pullup");
                return bp;
        }

        /*
         *  if not enough room in the first block,
         *  add another to the front of the list.
         */
        if (bp->lim - bp->rp < n) {
                nbp = block_alloc(n, MEM_WAIT);
                nbp->next = bp;
                bp = nbp;
        }

        /*
         *  copy bytes from the trailing blocks into the first
         */
        n -= BLEN(bp);
        while ((nbp = bp->next)) {
                i = BLEN(nbp);
                if (i > n) {
                        memmove(bp->wp, nbp->rp, n);
                        pullupblockcnt++;
                        bp->wp += n;
                        nbp->rp += n;
                        QDEBUG checkb(bp, "pullupblock 1");
                        return bp;
                } else {
                        memmove(bp->wp, nbp->rp, i);
                        pullupblockcnt++;
                        bp->wp += i;
                        bp->next = nbp->next;
                        nbp->next = 0;
                        freeb(nbp);
                        n -= i;
                        if (n == 0) {
                                QDEBUG checkb(bp, "pullupblock 2");
                                return bp;
                        }
                }
        }
        freeb(bp);
        return 0;
}

/*
 *  make sure the first block has at least n bytes in its main body
 */
struct block *pullupqueue(struct queue *q, int n)
{
        struct block *b;

        /* TODO: lock to protect the queue links? */
        if ((BHLEN(q->bfirst) >= n))
                return q->bfirst;
        q->bfirst = pullupblock(q->bfirst, n);
        for (b = q->bfirst; b != NULL && b->next != NULL; b = b->next) ;
        q->blast = b;
        return q->bfirst;
}

/* throw away count bytes from the front of
 * block's extradata.  Returns count of bytes
 * thrown away
 */

static int pullext(struct block *bp, int count)
{
        struct extra_bdata *ed;
        int i, rem, bytes = 0;

        for (i = 0; bp->extra_len && count && i < bp->nr_extra_bufs; i++) {
                ed = &bp->extra_data[i];
                rem = MIN(count, ed->len);
                bp->extra_len -= rem;
                count -= rem;
                bytes += rem;
                ed->off += rem;
                ed->len -= rem;
                if (ed->len == 0) {
                        kfree((void *)ed->base);
                        ed->base = 0;
                        ed->off = 0;
                }
        }
        return bytes;
}

/* throw away count bytes from the end of a
 * block's extradata.  Returns count of bytes
 * thrown away
 */

static int dropext(struct block *bp, int count)
{
        struct extra_bdata *ed;
        int i, rem, bytes = 0;

        for (i = bp->nr_extra_bufs - 1; bp->extra_len && count && i >= 0; i--) {
                ed = &bp->extra_data[i];
                rem = MIN(count, ed->len);
                bp->extra_len -= rem;
                count -= rem;
                bytes += rem;
                ed->len -= rem;
                if (ed->len == 0) {
                        kfree((void *)ed->base);
                        ed->base = 0;
                        ed->off = 0;
                }
        }
        return bytes;
}

/*
 *  throw away up to count bytes from a
 *  list of blocks.  Return count of bytes
 *  thrown away.
 */
static int _pullblock(struct block **bph, int count, int free)
{
        struct block *bp;
        int n, bytes;

        bytes = 0;
        if (bph == NULL)
                return 0;

        while (*bph != NULL && count != 0) {
                bp = *bph;

                n = MIN(BHLEN(bp), count);
                bytes += n;
                count -= n;
                bp->rp += n;
                n = pullext(bp, count);
                bytes += n;
                count -= n;
                QDEBUG checkb(bp, "pullblock ");
                if (BLEN(bp) == 0 && (free || count)) {
                        *bph = bp->next;
                        bp->next = NULL;
                        freeb(bp);
                }
        }
        return bytes;
}

int pullblock(struct block **bph, int count)
{
        return _pullblock(bph, count, 1);
}

/*
 *  trim to len bytes starting at offset
 */
struct block *trimblock(struct block *bp, int offset, int len)
{
        uint32_t l, trim;
        int olen = len;

        QDEBUG checkb(bp, "trimblock 1");
        if (blocklen(bp) < offset + len) {
                freeblist(bp);
                return NULL;
        }

        l =_pullblock(&bp, offset, 0);
        if (bp == NULL)
                return NULL;
        if (l != offset) {
                freeblist(bp);
                return NULL;
        }

        while ((l = BLEN(bp)) < len) {
                len -= l;
                bp = bp->next;
        }

        trim = BLEN(bp) - len;
        trim -= dropext(bp, trim);
        bp->wp -= trim;

        if (bp->next) {
                freeblist(bp->next);
                bp->next = NULL;
        }
        return bp;
}

/* Adjust block @bp so that its size is exactly @len.
 * If the size is increased, fill in the new contents with zeros.
 * If the size is decreased, discard some of the old contents at the tail. */
struct block *adjustblock(struct block *bp, int len)
{
        struct extra_bdata *ebd;
        void *buf;
        int i;

        if (len < 0) {
                freeb(bp);
                return NULL;
        }

        if (len == BLEN(bp))
                return bp;

        /* Shrink within block main body. */
        if (len <= BHLEN(bp)) {
                free_block_extra(bp);
                bp->wp = bp->rp + len;
                QDEBUG checkb(bp, "adjustblock 1");
                return bp;
        }

        /* Need to grow. */
        if (len > BLEN(bp)) {
                /* Grow within block main body. */
                if (bp->extra_len == 0 && bp->rp + len <= bp->lim) {
                        memset(bp->wp, 0, len - BLEN(bp));
                        bp->wp = bp->rp + len;
                        QDEBUG checkb(bp, "adjustblock 2");
                        return bp;
                }
                /* Grow with extra data buffers. */
                buf = kzmalloc(len - BLEN(bp), MEM_WAIT);
                block_append_extra(bp, (uintptr_t)buf, 0, len - BLEN(bp), MEM_WAIT);
                QDEBUG checkb(bp, "adjustblock 3");
                return bp;
        }

        /* Shrink extra data buffers.
         * len is how much of ebd we need to keep.
         * extra_len is re-accumulated. */
        assert(bp->extra_len > 0);
        len -= BHLEN(bp);
        bp->extra_len = 0;
        for (i = 0; i < bp->nr_extra_bufs; i++) {
                ebd = &bp->extra_data[i];
                if (len <= ebd->len)
                        break;
                len -= ebd->len;
                bp->extra_len += ebd->len;
        }
        /* If len becomes zero, extra_data[i] should be freed. */
        if (len > 0) {
                ebd = &bp->extra_data[i];
                ebd->len = len;
                bp->extra_len += ebd->len;
                i++;
        }
        for (; i < bp->nr_extra_bufs; i++) {
                ebd = &bp->extra_data[i];
                if (ebd->base)
                        kfree((void*)ebd->base);
                ebd->base = ebd->off = ebd->len = 0;
        }
        QDEBUG checkb(bp, "adjustblock 4");
        return bp;
}

/* Helper: removes and returns the first block from q */
static struct block *pop_first_block(struct queue *q)
{
        struct block *b = q->bfirst;

        q->len -= BALLOC(b);  // XXX all usages of q->len with extra_data are fucked
        q->dlen -= BLEN(b);
        q->bfirst = b->next;
        b->next = 0;
        return b;
}

/* Helper: copies up to copy_amt from a buf to a block's main body (b->wp) */
static size_t copy_to_block_body(struct block *to, void *from, size_t copy_amt)
{
        copy_amt = MIN(to->lim - to->wp, copy_amt);
        memcpy(to->wp, from, copy_amt);
        to->wp += copy_amt;
        return copy_amt;
}

/* Accounting helper.  Block b in q lost amt extra_data */
static void block_and_q_lost_extra(struct block *b, struct queue *q, size_t amt)
{
        b->extra_len -= amt;
        q->len -= amt;
        q->dlen -= amt;
}

/* Helper: moves ebd from a block (in from_q) to another block.  The *ebd is
 * fixed in 'from', so we move its contents and zero it out in 'from'.
 *
 * Returns the length moved (0 on failure). */
static size_t move_ebd(struct extra_bdata *ebd, struct block *to,
                       struct block *from, struct queue *from_q)
{
        size_t ret = ebd->len;

        if (block_append_extra(to, ebd->base, ebd->off, ebd->len, MEM_ATOMIC))
                return 0;
        block_and_q_lost_extra(from, from_q, ebd->len);
        ebd->base = ebd->len = ebd->off = 0;
        return ret;
}

/* Copy up to len bytes from q->bfirst to @to, leaving the block in place.  May
 * return with less than len, but greater than 0, even if there is more
 * available in q.
 *
 * At any moment that we have copied anything and things are tricky, we can just
 * return.  The trickiness comes from a bunch of variables: is the main body
 * empty?  How do we split the ebd?  If our alloc fails, then we can fall back
 * to @to's main body, but only if we haven't used it yet. */
static size_t copy_from_first_block(struct queue *q, struct block *to,
                                    size_t len)
{
        struct block *from = q->bfirst;
        size_t copy_amt, amt;
        struct extra_bdata *ebd;

        assert(len < BLEN(from));       /* sanity */
        /* Try to extract from the main body */
        copy_amt = MIN(BHLEN(from), len);
        if (copy_amt) {
                copy_amt = copy_to_block_body(to, from->rp, copy_amt);
                from->rp += copy_amt;
                /* We only change dlen, (data len), not q->len, since the q still has
                 * the same block memory allocation (no kfrees happened) */
                q->dlen -= copy_amt;
        }
        /* Try to extract the remainder from the extra data */
        len -= copy_amt;
        for (int i = 0; (i < from->nr_extra_bufs) && len; i++) {
                ebd = &from->extra_data[i];
                if (!ebd->base || !ebd->len)
                        continue;
                if (len >= ebd->len) {
                        amt = move_ebd(ebd, to, from, q);
                        if (!amt) {
                                /* our internal alloc could have failed.   this ebd is now the
                                 * last one we'll consider.  let's handle it separately and put
                                 * it in the main body. */
                                if (copy_amt)
                                        return copy_amt;
                                copy_amt = copy_to_block_body(to, (void*)ebd->base + ebd->off,
                                                              ebd->len);
                                block_and_q_lost_extra(from, q, copy_amt);
                                break;
                        }
                        len -= amt;
                        copy_amt += amt;
                        continue;
                } else {
                        /* If we're here, we reached our final ebd, which we'll need to
                         * split to get anything from it. */
                        if (copy_amt)
                                return copy_amt;
                        copy_amt = copy_to_block_body(to, (void*)ebd->base + ebd->off,
                                                      len);
                        ebd->off += copy_amt;
                        ebd->len -= copy_amt;
                        block_and_q_lost_extra(from, q, copy_amt);
                        break;
                }
        }
        if (len)
                assert(copy_amt);       /* sanity */
        return copy_amt;
}

/* Return codes for __qbread and __try_qbread. */
enum {
        QBR_OK,
        QBR_FAIL,
        QBR_SPARE,      /* we need a spare block */
        QBR_AGAIN,      /* do it again, we are coalescing blocks */
};

/* Helper and back-end for __qbread: extracts and returns a list of blocks
 * containing up to len bytes.  It may contain less than len even if q has more
 * data.
 *
 * Returns a code interpreted by __qbread, and the returned blist in ret. */
static int __try_qbread(struct queue *q, size_t len, int qio_flags,
                        struct block **real_ret, struct block *spare)
{
        struct block *ret, *ret_last, *first;
        size_t blen;
        bool was_unwritable = FALSE;
        int dowakeup = 0;

        if (qio_flags & QIO_CAN_ERR_SLEEP) {
                if (!qwait_and_ilock(q, qio_flags)) {
                        spin_unlock_irqsave(&q->lock);
                        return QBR_FAIL;
                }
                /* we qwaited and still hold the lock, so the q is not empty */
                first = q->bfirst;
        } else {
                spin_lock_irqsave(&q->lock);
                first = q->bfirst;
                if (!first) {
                        spin_unlock_irqsave(&q->lock);
                        return QBR_FAIL;
                }
        }
        was_unwritable = !qwritable(q);
        blen = BLEN(first);
        if ((q->state & Qcoalesce) && (blen == 0)) {
                freeb(pop_first_block(q));
                spin_unlock_irqsave(&q->lock);
                /* Need to retry to make sure we have a first block */
                return QBR_AGAIN;
        }
        /* Qmsg: just return the first block.  Be careful, since our caller might
         * not read all of the block and thus drop bytes.  Similar to SOCK_DGRAM. */
        if (q->state & Qmsg) {
                ret = pop_first_block(q);
                goto out_ok;
        }
        /* Let's get at least something first - makes the code easier.  This way,
         * we'll only ever split the block once. */
        if (blen <= len) {
                ret = pop_first_block(q);
                len -= blen;
        } else {
                /* need to split the block.  we won't actually take the first block out
                 * of the queue - we're just extracting a little bit. */
                if (!spare) {
                        /* We have nothing and need a spare block.  Retry! */
                        spin_unlock_irqsave(&q->lock);
                        return QBR_SPARE;
                }
                copy_from_first_block(q, spare, len);
                ret = spare;
                goto out_ok;
        }
        /* At this point, we just grabbed the first block.  We can try to grab some
         * more, up to len (if they want). */
        if (qio_flags & QIO_JUST_ONE_BLOCK)
                goto out_ok;
        ret_last = ret;
        while (q->bfirst && (len > 0)) {
                blen = BLEN(q->bfirst);
                if ((q->state & Qcoalesce) && (blen == 0)) {
                        /* remove the intermediate 0 blocks */
                        freeb(pop_first_block(q));
                        continue;
                }
                if (blen > len) {
                        /* We could try to split the block, but that's a huge pain.  For
                         * instance, we might need to move the main body of b into an
                         * extra_data of ret_last.  lots of ways for that to fail, and lots
                         * of cases to consider.  Easier to just bail out.  This is why I
                         * did the first block above: we don't need to worry about this. */
                         break;
                }
                ret_last->next = pop_first_block(q);
                ret_last = ret_last->next;
                len -= blen;
        }
out_ok:
        /*
         *  if writer flow controlled, restart
         *
         *  This used to be
         *  q->len < q->limit/2
         *  but it slows down tcp too much for certain write sizes.
         *  I really don't understand it completely.  It may be
         *  due to the queue draining so fast that the transmission
         *  stalls waiting for the app to produce more data.  - presotto
         */
        if ((q->state & Qflow) && q->len < q->limit) {
                q->state &= ~Qflow;
                dowakeup = 1;
        }
        spin_unlock_irqsave(&q->lock);
        /* wakeup flow controlled writers */
        if (dowakeup) {
                if (q->kick && !(qio_flags & QIO_DONT_KICK))
                        q->kick(q->arg);
                rendez_wakeup(&q->wr);
        }
        if (was_unwritable)
                qwake_cb(q, FDTAP_FILT_WRITABLE);
        *real_ret = ret;
        return QBR_OK;
}

/* Helper and front-end for __try_qbread: extracts and returns a list of blocks
 * containing up to len bytes.  It may contain less than len even if q has more
 * data.
 *
 * Returns 0 if the q is closed or would require blocking and !CAN_BLOCK.
 *
 * Technically, there's a weird corner case with !Qcoalesce and Qmsg where you
 * could get a zero length block back. */
static struct block *__qbread(struct queue *q, size_t len, int qio_flags,
                              int mem_flags)
{
        struct block *ret = 0;
        struct block *spare = 0;

        while (1) {
                switch (__try_qbread(q, len, qio_flags, &ret, spare)) {
                case QBR_OK:
                case QBR_FAIL:
                        if (spare && (ret != spare))
                                freeb(spare);
                        return ret;
                case QBR_SPARE:
                        assert(!spare);
                        /* Due to some nastiness, we need a fresh block so we can read out
                         * anything from the queue.  'len' seems like a reasonable amount.
                         * Maybe we can get away with less. */
                        spare = block_alloc(len, mem_flags);
                        if (!spare)
                                return 0;
                        break;
                case QBR_AGAIN:
                        /* if the first block is 0 and we are Qcoalesce, then we'll need to
                         * try again.  We bounce out of __try so we can perform the "is
                         * there a block" logic again from the top. */
                        break;
                }
        }
}

/*
 *  get next block from a queue, return null if nothing there
 */
struct block *qget(struct queue *q)
{
        /* since len == SIZE_MAX, we should never need to do a mem alloc */
        return __qbread(q, SIZE_MAX, QIO_JUST_ONE_BLOCK, MEM_ATOMIC);
}

/* Throw away the next 'len' bytes in the queue returning the number actually
 * discarded.
 *
 * If the bytes are in the queue, then they must be discarded.  The only time to
 * return less than len is if the q itself has less than len bytes.
 *
 * This won't trigger a kick when waking up any sleepers.  This seems to be Plan
 * 9's intent, since the TCP stack will deadlock if qdiscard kicks. */
size_t qdiscard(struct queue *q, size_t len)
{
        struct block *blist;
        size_t removed_amt;
        size_t sofar = 0;

        /* This is racy.  There could be multiple qdiscarders or other consumers,
         * where the consumption could be interleaved. */
        while (qlen(q) && len) {
                blist = __qbread(q, len, QIO_DONT_KICK, MEM_WAIT);
                removed_amt = freeblist(blist);
                sofar += removed_amt;
                len -= removed_amt;
        }
        return sofar;
}

ssize_t qpass(struct queue *q, struct block *b)
{
        return __qbwrite(q, b, QIO_LIMIT | QIO_DROP_OVERFLOW);
}

ssize_t qpassnolim(struct queue *q, struct block *b)
{
        return __qbwrite(q, b, 0);
}

/*
 *  if the allocated space is way out of line with the used
 *  space, reallocate to a smaller block
 */
struct block *packblock(struct block *bp)
{
        struct block **l, *nbp;
        int n;

        if (bp->extra_len)
                return bp;
        for (l = &bp; *l; l = &(*l)->next) {
                nbp = *l;
                n = BLEN(nbp);
                if ((n << 2) < BALLOC(nbp)) {
                        *l = block_alloc(n, MEM_WAIT);
                        memmove((*l)->wp, nbp->rp, n);
                        (*l)->wp += n;
                        (*l)->next = nbp->next;
                        freeb(nbp);
                }
        }

        return bp;
}

/* Add an extra_data entry to newb at newb_idx pointing to b's body, starting at
 * body_rp, for up to len.  Returns the len consumed.
 *
 * The base is 'b', so that we can kfree it later.  This currently ties us to
 * using kfree for the release method for all extra_data.
 *
 * It is possible to have a body size that is 0, if there is no offset, and
 * b->wp == b->rp.  This will have an extra data entry of 0 length. */
static size_t point_to_body(struct block *b, uint8_t *body_rp,
                            struct block *newb, unsigned int newb_idx,
                            size_t len)
{
        struct extra_bdata *ebd = &newb->extra_data[newb_idx];

        assert(newb_idx < newb->nr_extra_bufs);

        kmalloc_incref(b);
        ebd->base = (uintptr_t)b;
        ebd->off = (uint32_t)(body_rp - (uint8_t*)b);
        ebd->len = MIN(b->wp - body_rp, len);   /* think of body_rp as b->rp */
        assert((int)ebd->len >= 0);
        newb->extra_len += ebd->len;
        return ebd->len;
}

/* Add an extra_data entry to newb at newb_idx pointing to b's b_idx'th
 * extra_data buf, at b_off within that buffer, for up to len.  Returns the len
 * consumed.
 *
 * We can have blocks with 0 length, but they are still refcnt'd.  See above. */
static size_t point_to_buf(struct block *b, unsigned int b_idx, uint32_t b_off,
                           struct block *newb, unsigned int newb_idx,
                           size_t len)
{
        struct extra_bdata *n_ebd = &newb->extra_data[newb_idx];
        struct extra_bdata *b_ebd = &b->extra_data[b_idx];

        assert(b_idx < b->nr_extra_bufs);
        assert(newb_idx < newb->nr_extra_bufs);

        kmalloc_incref((void*)b_ebd->base);
        n_ebd->base = b_ebd->base;
        n_ebd->off = b_ebd->off + b_off;
        n_ebd->len = MIN(b_ebd->len - b_off, len);
        newb->extra_len += n_ebd->len;
        return n_ebd->len;
}

/* given a string of blocks, sets up the new block's extra_data such that it
 * *points* to the contents of the blist [offset, len + offset).  This does not
 * make a separate copy of the contents of the blist.
 *
 * returns 0 on success.  the only failure is if the extra_data array was too
 * small, so this returns a positive integer saying how big the extra_data needs
 * to be.
 *
 * callers are responsible for protecting the list structure. */
static int __blist_clone_to(struct block *blist, struct block *newb, int len,
                            uint32_t offset)
{
        struct block *b, *first;
        unsigned int nr_bufs = 0;
        unsigned int b_idx, newb_idx = 0;
        uint8_t *first_main_body = 0;

        /* find the first block; keep offset relative to the latest b in the list */
        for (b = blist; b; b = b->next) {
                if (BLEN(b) > offset)
                        break;
                offset -= BLEN(b);
        }
        /* qcopy semantics: if you asked for an offset outside the block list, you
         * get an empty block back */
        if (!b)
                return 0;
        first = b;
        /* upper bound for how many buffers we'll need in newb */
        for (/* b is set*/; b; b = b->next) {
                nr_bufs += 1 + b->nr_extra_bufs;        /* 1 for the main body */
        }
        /* we might be holding a spinlock here, so we won't wait for kmalloc */
        if (block_add_extd(newb, nr_bufs, 0) != 0) {
                /* caller will need to alloc these, then re-call us */
                return nr_bufs;
        }
        for (b = first; b && len; b = b->next) {
                b_idx = 0;
                if (offset) {
                        if (offset < BHLEN(b)) {
                                /* off is in the main body */
                                len -= point_to_body(b, b->rp + offset, newb, newb_idx, len);
                                newb_idx++;
                        } else {
                                /* off is in one of the buffers (or just past the last one).
                                 * we're not going to point to b's main body at all. */
                                offset -= BHLEN(b);
                                assert(b->extra_data);
                                /* assuming these extrabufs are packed, or at least that len
                                 * isn't gibberish */
                                while (b->extra_data[b_idx].len <= offset) {
                                        offset -= b->extra_data[b_idx].len;
                                        b_idx++;
                                }
                                /* now offset is set to our offset in the b_idx'th buf */
                                len -= point_to_buf(b, b_idx, offset, newb, newb_idx, len);
                                newb_idx++;
                                b_idx++;
                        }
                        offset = 0;
                } else {
                        len -= point_to_body(b, b->rp, newb, newb_idx, len);
                        newb_idx++;
                }
                /* knock out all remaining bufs.  we only did one point_to_ op by now,
                 * and any point_to_ could be our last if it consumed all of len. */
                for (int i = b_idx; (i < b->nr_extra_bufs) && len; i++) {
                        len -= point_to_buf(b, i, 0, newb, newb_idx, len);
                        newb_idx++;
                }
        }
        return 0;
}

struct block *blist_clone(struct block *blist, int header_len, int len,
                          uint32_t offset)
{
        int ret;
        struct block *newb = block_alloc(header_len, MEM_WAIT);
        do {
                ret = __blist_clone_to(blist, newb, len, offset);
                if (ret)
                        block_add_extd(newb, ret, MEM_WAIT);
        } while (ret);
        return newb;
}

/* given a queue, makes a single block with header_len reserved space in the
 * block main body, and the contents of [offset, len + offset) pointed to in the
 * new blocks ext_data.  This does not make a copy of the q's contents, though
 * you do have a ref count on the memory. */
struct block *qclone(struct queue *q, int header_len, int len, uint32_t offset)
{
        int ret;
        struct block *newb = block_alloc(header_len, MEM_WAIT);
        /* the while loop should rarely be used: it would require someone
         * concurrently adding to the queue. */
        do {
                /* TODO: RCU: protecting the q list (b->next) (need read lock) */
                spin_lock_irqsave(&q->lock);
                ret = __blist_clone_to(q->bfirst, newb, len, offset);
                spin_unlock_irqsave(&q->lock);
                if (ret)
                        block_add_extd(newb, ret, MEM_WAIT);
        } while (ret);
        return newb;
}

/*
 *  copy from offset in the queue
 */
struct block *qcopy_old(struct queue *q, int len, uint32_t offset)
{
        int sofar;
        int n;
        struct block *b, *nb;
        uint8_t *p;

        nb = block_alloc(len, MEM_WAIT);

        spin_lock_irqsave(&q->lock);

        /* go to offset */
        b = q->bfirst;
        for (sofar = 0;; sofar += n) {
                if (b == NULL) {
                        spin_unlock_irqsave(&q->lock);
                        return nb;
                }
                n = BLEN(b);
                if (sofar + n > offset) {
                        p = b->rp + offset - sofar;
                        n -= offset - sofar;
                        break;
                }
                QDEBUG checkb(b, "qcopy");
                b = b->next;
        }

        /* copy bytes from there */
        for (sofar = 0; sofar < len;) {
                if (n > len - sofar)
                        n = len - sofar;
                PANIC_EXTRA(b);
                memmove(nb->wp, p, n);
                qcopycnt += n;
                sofar += n;
                nb->wp += n;
                b = b->next;
                if (b == NULL)
                        break;
                n = BLEN(b);
                p = b->rp;
        }
        spin_unlock_irqsave(&q->lock);

        return nb;
}

struct block *qcopy(struct queue *q, int len, uint32_t offset)
{
#ifdef CONFIG_BLOCK_EXTRAS
        return qclone(q, 0, len, offset);
#else
        return qcopy_old(q, len, offset);
#endif
}

static void qinit_common(struct queue *q)
{
        spinlock_init_irqsave(&q->lock);
        rendez_init(&q->rr);
        rendez_init(&q->wr);
}

/*
 *  called by non-interrupt code
 */
struct queue *qopen(int limit, int msg, void (*kick) (void *), void *arg)
{
        struct queue *q;

        q = kzmalloc(sizeof(struct queue), 0);
        if (q == 0)
                return 0;
        qinit_common(q);

        q->limit = q->inilim = limit;
        q->kick = kick;
        q->arg = arg;
        q->state = msg;
        q->state |= Qstarve;
        q->eof = 0;

        return q;
}

/* open a queue to be bypassed */
struct queue *qbypass(void (*bypass) (void *, struct block *), void *arg)
{
        struct queue *q;

        q = kzmalloc(sizeof(struct queue), 0);
        if (q == 0)
                return 0;
        qinit_common(q);

        q->limit = 0;
        q->arg = arg;
        q->bypass = bypass;
        q->state = 0;

        return q;
}

static int notempty(void *a)
{
        struct queue *q = a;

        return (q->state & Qclosed) || q->bfirst != 0;
}

/* Block, waiting for the queue to be non-empty or closed.  Returns with
 * the spinlock held.  Returns TRUE when there queue is not empty, FALSE if it
 * was naturally closed.  Throws an error o/w. */
static bool qwait_and_ilock(struct queue *q, int qio_flags)
{
        while (1) {
                spin_lock_irqsave(&q->lock);
                if (q->bfirst != NULL)
                        return TRUE;
                if (q->state & Qclosed) {
                        if (++q->eof > 3) {
                                spin_unlock_irqsave(&q->lock);
                                error(EPIPE, "multiple reads on a closed queue");
                        }
                        if (q->err[0]) {
                                spin_unlock_irqsave(&q->lock);
                                error(EPIPE, q->err);
                        }
                        return FALSE;
                }
                /* We set Qstarve regardless of whether we are non-blocking or not.
                 * Qstarve tracks the edge detection of the queue being empty. */
                q->state |= Qstarve;
                if (qio_flags & QIO_NON_BLOCK) {
                        spin_unlock_irqsave(&q->lock);
                        error(EAGAIN, "queue empty");
                }
                spin_unlock_irqsave(&q->lock);
                /* may throw an error() */
                rendez_sleep(&q->rr, notempty, q);
        }
}

/*
 * add a block list to a queue
 * XXX basically the same as enqueue blist, and has no locking!
 */
void qaddlist(struct queue *q, struct block *b)
{
        /* TODO: q lock? */
        /* queue the block */
        if (q->bfirst)
                q->blast->next = b;
        else
                q->bfirst = b;
        q->len += blockalloclen(b);
        q->dlen += blocklen(b);
        while (b->next)
                b = b->next;
        q->blast = b;
}

static size_t read_from_block(struct block *b, uint8_t *to, size_t amt)
{
        size_t copy_amt, retval = 0;
        struct extra_bdata *ebd;

        copy_amt = MIN(BHLEN(b), amt);
        memcpy(to, b->rp, copy_amt);
        /* advance the rp, since this block not be completely consumed and future
         * reads need to know where to pick up from */
        b->rp += copy_amt;
        to += copy_amt;
        amt -= copy_amt;
        retval += copy_amt;
        for (int i = 0; (i < b->nr_extra_bufs) && amt; i++) {
                ebd = &b->extra_data[i];
                /* skip empty entires.  if we track this in the struct block, we can
                 * just start the for loop early */
                if (!ebd->base || !ebd->len)
                        continue;
                copy_amt = MIN(ebd->len, amt);
                memcpy(to, (void*)(ebd->base + ebd->off), copy_amt);
                /* we're actually consuming the entries, just like how we advance rp up
                 * above, and might only consume part of one. */
                ebd->len -= copy_amt;
                ebd->off += copy_amt;
                b->extra_len -= copy_amt;
                if (!ebd->len) {
                        /* we don't actually have to decref here.  it's also done in
                         * freeb().  this is the earliest we can free. */
                        kfree((void*)ebd->base);
                        ebd->base = ebd->off = 0;
                }
                to += copy_amt;
                amt -= copy_amt;
                retval += copy_amt;
        }
        return retval;
}

/*
 *  copy the contents of a string of blocks into
 *  memory.  emptied blocks are freed.  return
 *  pointer to first unconsumed block.
 */
struct block *bl2mem(uint8_t * p, struct block *b, int n)
{
        int i;
        struct block *next;

        /* could be slicker here, since read_from_block is smart */
        for (; b != NULL; b = next) {
                i = BLEN(b);
                if (i > n) {
                        /* partial block, consume some */
                        read_from_block(b, p, n);
                        return b;
                }
                /* full block, consume all and move on */
                i = read_from_block(b, p, i);
                n -= i;
                p += i;
                next = b->next;
                freeb(b);
        }
        return NULL;
}

/* Extract the contents of all blocks and copy to va, up to len.  Returns the
 * actual amount copied. */
static size_t read_all_blocks(struct block *b, void *va, size_t len)
{
        size_t sofar = 0;
        struct block *next;

        do {
                /* We should be draining every block completely. */
                assert(BLEN(b) <= len - sofar);
                assert(va);
                assert(va + sofar);
                assert(b->rp);
                sofar += read_from_block(b, va + sofar, len - sofar);
                next = b->next;
                freeb(b);
                b = next;
        } while (b);
        return sofar;
}

/*
 *  copy the contents of memory into a string of blocks.
 *  return NULL on error.
 */
struct block *mem2bl(uint8_t * p, int len)
{
        ERRSTACK(1);
        int n;
        struct block *b, *first, **l;

        first = NULL;
        l = &first;
        if (waserror()) {
                freeblist(first);
                nexterror();
        }
        do {
                n = len;
                if (n > Maxatomic)
                        n = Maxatomic;

                *l = b = block_alloc(n, MEM_WAIT);
                /* TODO consider extra_data */
                memmove(b->wp, p, n);
                b->wp += n;
                p += n;
                len -= n;
                l = &b->next;
        } while (len > 0);
        poperror();

        return first;
}

/*
 *  put a block back to the front of the queue
 *  called with q ilocked
 */
void qputback(struct queue *q, struct block *b)
{
        b->next = q->bfirst;
        if (q->bfirst == NULL)
                q->blast = b;
        q->bfirst = b;
        q->len += BALLOC(b);
        q->dlen += BLEN(b);
}

/*
 *  get next block from a queue (up to a limit)
 *
 */
struct block *qbread(struct queue *q, size_t len)
{
        return __qbread(q, len, QIO_JUST_ONE_BLOCK | QIO_CAN_ERR_SLEEP, MEM_WAIT);
}

struct block *qbread_nonblock(struct queue *q, size_t len)
{
        return __qbread(q, len, QIO_JUST_ONE_BLOCK | QIO_CAN_ERR_SLEEP |
                        QIO_NON_BLOCK, MEM_WAIT);
}

/* read up to len from a queue into vp. */
size_t qread(struct queue *q, void *va, size_t len)
{
        struct block *blist = __qbread(q, len, QIO_CAN_ERR_SLEEP, MEM_WAIT);

        if (!blist)
                return 0;
        return read_all_blocks(blist, va, len);
}

size_t qread_nonblock(struct queue *q, void *va, size_t len)
{
        struct block *blist = __qbread(q, len, QIO_CAN_ERR_SLEEP | QIO_NON_BLOCK,
                                       MEM_WAIT);

        if (!blist)
                return 0;
        return read_all_blocks(blist, va, len);
}

static int qnotfull(void *a)
{
        struct queue *q = a;

        return q->len < q->limit || (q->state & Qclosed);
}

/* Helper: enqueues a list of blocks to a queue.  Returns the total length. */
static size_t enqueue_blist(struct queue *q, struct block *b)
{
        size_t len, dlen;

        if (q->bfirst)
                q->blast->next = b;
        else
                q->bfirst = b;
        len = BALLOC(b);
        dlen = BLEN(b);
        while (b->next) {
                b = b->next;
                len += BALLOC(b);
                dlen += BLEN(b);
        }
        q->blast = b;
        q->len += len;
        q->dlen += dlen;
        return dlen;
}

/* Adds block (which can be a list of blocks) to the queue, subject to
 * qio_flags.  Returns the length written on success or -1 on non-throwable
 * error.  Adjust qio_flags to control the value-added features!. */
static ssize_t __qbwrite(struct queue *q, struct block *b, int qio_flags)
{
        ssize_t ret;
        bool dowakeup = FALSE;
        bool was_empty;

        if (q->bypass) {
                ret = blocklen(b);
                (*q->bypass) (q->arg, b);
                return ret;
        }
        spin_lock_irqsave(&q->lock);
        was_empty = q->len == 0;
        if (q->state & Qclosed) {
                spin_unlock_irqsave(&q->lock);
                freeblist(b);
                if (!(qio_flags & QIO_CAN_ERR_SLEEP))
                        return -1;
                if (q->err[0])
                        error(EPIPE, q->err);
                else
                        error(EPIPE, "connection closed");
        }
        if ((qio_flags & QIO_LIMIT) && (q->len >= q->limit)) {
                /* drop overflow takes priority over regular non-blocking */
                if ((qio_flags & QIO_DROP_OVERFLOW) || (q->state & Qdropoverflow)) {
                        spin_unlock_irqsave(&q->lock);
                        freeb(b);
                        return -1;
                }
                /* People shouldn't set NON_BLOCK without CAN_ERR, but we can be nice
                 * and catch it. */
                if ((qio_flags & QIO_CAN_ERR_SLEEP) && (qio_flags & QIO_NON_BLOCK)) {
                        spin_unlock_irqsave(&q->lock);
                        freeb(b);
                        error(EAGAIN, "queue full");
                }
        }
        ret = enqueue_blist(q, b);
        QDEBUG checkb(b, "__qbwrite");
        /* make sure other end gets awakened */
        if (q->state & Qstarve) {
                q->state &= ~Qstarve;
                dowakeup = TRUE;
        }
        spin_unlock_irqsave(&q->lock);
        /* TODO: not sure if the usage of a kick is mutually exclusive with a
         * wakeup, meaning that actual users either want a kick or have qreaders. */
        if (q->kick && (dowakeup || (q->state & Qkick)))
                q->kick(q->arg);
        if (dowakeup)
                rendez_wakeup(&q->rr);
        if (was_empty)
                qwake_cb(q, FDTAP_FILT_READABLE);
        /*
         *  flow control, wait for queue to get below the limit
         *  before allowing the process to continue and queue
         *  more.  We do this here so that postnote can only
         *  interrupt us after the data has been queued.  This
         *  means that things like 9p flushes and ssl messages
         *  will not be disrupted by software interrupts.
         *
         *  Note - this is moderately dangerous since a process
         *  that keeps getting interrupted and rewriting will
         *  queue infinite crud.
         */
        if ((qio_flags & QIO_CAN_ERR_SLEEP) &&
            !(q->state & Qdropoverflow) && !(qio_flags & QIO_NON_BLOCK)) {
                /* This is a racy peek at the q status.  If we accidentally block, we
                 * set Qflow, so someone should wake us.  If we accidentally don't
                 * block, we just returned to the user and let them slip a block past
                 * flow control. */
                while (!qnotfull(q)) {
                        spin_lock_irqsave(&q->lock);
                        q->state |= Qflow;
                        spin_unlock_irqsave(&q->lock);
                        rendez_sleep(&q->wr, qnotfull, q);
                }
        }
        return ret;
}

/*
 *  add a block to a queue obeying flow control
 */
ssize_t qbwrite(struct queue *q, struct block *b)
{
        return __qbwrite(q, b, QIO_CAN_ERR_SLEEP | QIO_LIMIT);
}

ssize_t qbwrite_nonblock(struct queue *q, struct block *b)
{
        return __qbwrite(q, b, QIO_CAN_ERR_SLEEP | QIO_LIMIT | QIO_NON_BLOCK);
}

ssize_t qibwrite(struct queue *q, struct block *b)
{
        return __qbwrite(q, b, 0);
}

/* Helper, allocs a block and copies [from, from + len) into it.  Returns the
 * block on success, 0 on failure. */
static struct block *build_block(void *from, size_t len, int mem_flags)
{
        struct block *b;
        void *ext_buf;

        /* If len is small, we don't need to bother with the extra_data.  But until
         * the whole stack can handle extd blocks, we'll use them unconditionally.
         * */
#ifdef CONFIG_BLOCK_EXTRAS
        /* allocb builds in 128 bytes of header space to all blocks, but this is
         * only available via padblock (to the left).  we also need some space
         * for pullupblock for some basic headers (like icmp) that get written
         * in directly */
        b = block_alloc(64, mem_flags);
        if (!b)
                return 0;
        ext_buf = kmalloc(len, mem_flags);
        if (!ext_buf) {
                kfree(b);
                return 0;
        }
        memcpy(ext_buf, from, len);
        if (block_add_extd(b, 1, mem_flags)) {
                kfree(ext_buf);
                kfree(b);
                return 0;
        }
        b->extra_data[0].base = (uintptr_t)ext_buf;
        b->extra_data[0].off = 0;
        b->extra_data[0].len = len;
        b->extra_len += len;
#else
        b = block_alloc(len, mem_flags);
        if (!b)
                return 0;
        memmove(b->wp, from, len);
        b->wp += len;
#endif
        return b;
}

static ssize_t __qwrite(struct queue *q, void *vp, size_t len, int mem_flags,
                        int qio_flags)
{
        size_t n, sofar;
        struct block *b;
        uint8_t *p = vp;
        void *ext_buf;

        sofar = 0;
        do {
                n = len - sofar;
                /* This is 64K, the max amount per single block.  Still a good value? */
                if (n > Maxatomic)
                        n = Maxatomic;
                b = build_block(p + sofar, n, mem_flags);
                if (!b)
                        break;
                if (__qbwrite(q, b, qio_flags) < 0)
                        break;
                sofar += n;
        } while ((sofar < len) && (q->state & Qmsg) == 0);
        return sofar;
}

ssize_t qwrite(struct queue *q, void *vp, int len)
{
        return __qwrite(q, vp, len, MEM_WAIT, QIO_CAN_ERR_SLEEP | QIO_LIMIT);
}

ssize_t qwrite_nonblock(struct queue *q, void *vp, int len)
{
        return __qwrite(q, vp, len, MEM_WAIT, QIO_CAN_ERR_SLEEP | QIO_LIMIT |
                                              QIO_NON_BLOCK);
}

ssize_t qiwrite(struct queue *q, void *vp, int len)
{
        return __qwrite(q, vp, len, MEM_ATOMIC, 0);
}

/*
 *  be extremely careful when calling this,
 *  as there is no reference accounting
 */
void qfree(struct queue *q)
{
        qclose(q);
        kfree(q);
}

/*
 *  Mark a queue as closed.  No further IO is permitted.
 *  All blocks are released.
 */
void qclose(struct queue *q)
{
        struct block *bfirst;

        if (q == NULL)
                return;

        /* mark it */
        spin_lock_irqsave(&q->lock);
        q->state |= Qclosed;
        q->state &= ~(Qflow | Qstarve | Qdropoverflow);
        q->err[0] = 0;
        bfirst = q->bfirst;
        q->bfirst = 0;
        q->len = 0;
        q->dlen = 0;
        spin_unlock_irqsave(&q->lock);

        /* free queued blocks */
        freeblist(bfirst);

        /* wake up readers/writers */
        rendez_wakeup(&q->rr);
        rendez_wakeup(&q->wr);
        qwake_cb(q, FDTAP_FILT_HANGUP);
}

/* Mark a queue as closed.  Wakeup any readers.  Don't remove queued blocks.
 *
 * msg will be the errstr received by any waiters (qread, qbread, etc).  If
 * there is no message, which is what also happens during a natural qclose(),
 * those waiters will simply return 0.  qwriters will always error() on a
 * closed/hungup queue. */
void qhangup(struct queue *q, char *msg)
{
        /* mark it */
        spin_lock_irqsave(&q->lock);
        q->state |= Qclosed;
        if (msg == 0 || *msg == 0)
                q->err[0] = 0;
        else
                strlcpy(q->err, msg, ERRMAX);
        spin_unlock_irqsave(&q->lock);

        /* wake up readers/writers */
        rendez_wakeup(&q->rr);
        rendez_wakeup(&q->wr);
        qwake_cb(q, FDTAP_FILT_HANGUP);
}

/*
 *  return non-zero if the q is hungup
 */
int qisclosed(struct queue *q)
{
        return q->state & Qclosed;
}

/*
 *  mark a queue as no longer hung up.  resets the wake_cb.
 */
void qreopen(struct queue *q)
{
        spin_lock_irqsave(&q->lock);
        q->state &= ~Qclosed;
        q->state |= Qstarve;
        q->eof = 0;
        q->limit = q->inilim;
        q->wake_cb = 0;
        q->wake_data = 0;
        spin_unlock_irqsave(&q->lock);
}

/*
 *  return bytes queued
 */
int qlen(struct queue *q)
{
        return q->dlen;
}

/*
 * return space remaining before flow control
 */
int qwindow(struct queue *q)
{
        int l;

        l = q->limit - q->len;
        if (l < 0)
                l = 0;
        return l;
}

/*
 *  return true if we can read without blocking
 */
int qcanread(struct queue *q)
{
        return q->bfirst != 0;
}

/*
 *  change queue limit
 */
void qsetlimit(struct queue *q, int limit)
{
        q->limit = limit;
}

/*
 *  set whether writes drop overflowing blocks, or if we sleep
 */
void qdropoverflow(struct queue *q, bool onoff)
{
        spin_lock_irqsave(&q->lock);
        if (onoff)
                q->state |= Qdropoverflow;
        else
                q->state &= ~Qdropoverflow;
        spin_unlock_irqsave(&q->lock);
}

/* Be careful: this can affect concurrent reads/writes and code that might have
 * built-in expectations of the q's type. */
void q_toggle_qmsg(struct queue *q, bool onoff)
{
        spin_lock_irqsave(&q->lock);
        if (onoff)
                q->state |= Qmsg;
        else
                q->state &= ~Qmsg;
        spin_unlock_irqsave(&q->lock);
}

/* Be careful: this can affect concurrent reads/writes and code that might have
 * built-in expectations of the q's type. */
void q_toggle_qcoalesce(struct queue *q, bool onoff)
{
        spin_lock_irqsave(&q->lock);
        if (onoff)
                q->state |= Qcoalesce;
        else
                q->state &= ~Qcoalesce;
        spin_unlock_irqsave(&q->lock);
}

/*
 *  flush the output queue
 */
void qflush(struct queue *q)
{
        struct block *bfirst;

        /* mark it */
        spin_lock_irqsave(&q->lock);
        bfirst = q->bfirst;
        q->bfirst = 0;
        q->len = 0;
        q->dlen = 0;
        spin_unlock_irqsave(&q->lock);

        /* free queued blocks */
        freeblist(bfirst);

        /* wake up writers */
        rendez_wakeup(&q->wr);
        qwake_cb(q, FDTAP_FILT_WRITABLE);
}

int qfull(struct queue *q)
{
        return q->len >= q->limit;
}

int qstate(struct queue *q)
{
        return q->state;
}

void qdump(struct queue *q)
{
        if (q)
                printk("q=%p bfirst=%p blast=%p len=%d dlen=%d limit=%d state=#%x\n",
                           q, q->bfirst, q->blast, q->len, q->dlen, q->limit, q->state);
}

/* On certain wakeup events, qio will call func(q, data, filter), where filter
 * marks the type of wakeup event (flags from FDTAP).
 *
 * There's no sync protection.  If you change the CB while the qio is running,
 * you might get a CB with the data or func from a previous set_wake_cb.  You
 * should set this once per queue and forget it.
 *
 * You can remove the CB by passing in 0 for the func.  Alternatively, you can
 * just make sure that the func(data) pair are valid until the queue is freed or
 * reopened. */
void qio_set_wake_cb(struct queue *q, qio_wake_cb_t func, void *data)
{
        q->wake_data = data;
        wmb();  /* if we see func, we'll also see the data for it */
        q->wake_cb = func;
}

/* Helper for detecting whether we'll block on a read at this instant. */
bool qreadable(struct queue *q)
{
        return qlen(q) > 0;
}

/* Helper for detecting whether we'll block on a write at this instant. */
bool qwritable(struct queue *q)
{
        return qwindow(q) > 0;
}