Commit 88266f5a authored by Stefan Hajnoczi's avatar Stefan Hajnoczi
Browse files

block: stop relying on io_flush() in bdrv_drain_all() 



If a block driver has no file descriptors to monitor but there are still
active requests, it can return 1 from .io_flush().  This is used to spin
during synchronous I/O.

Stop relying on .io_flush() and instead check
QLIST_EMPTY(&bs->tracked_requests) to decide whether there are active
requests.

This is the first step in removing .io_flush() so that event loops no
longer need to have the concept of synchronous I/O.  Eventually we may
be able to kill synchronous I/O completely by running everything in a
coroutine, but that is future work.

Note this patch moves bs->throttled_reqs initialization to bdrv_new() so
that bdrv_requests_pending(bs) can safely access it.  In practice bs is
g_malloc0() so the memory is already zeroed but it's safer to initialize
the queue properly.

We also need to fix up block/stream.c:close_unused_images() to prevent
traversing a dangling pointer while it rearranges the backing file
chain.  This is necessary since the new bdrv_drain_all() traverses the
backing file chain.

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
parent e1b5c52e

block.c

+35 −10
Original line number Diff line number Diff line
@@ -148,7 +148,6 @@ static void bdrv_block_timer(void *opaque) 


void bdrv_io_limits_enable(BlockDriverState *bs)

{

    qemu_co_queue_init(&bs->throttled_reqs);

    bs->block_timer = qemu_new_timer_ns(vm_clock, bdrv_block_timer, bs);

    bs->io_limits_enabled = true;

}
@@ -306,6 +305,7 @@ BlockDriverState *bdrv_new(const char *device_name) 
    bdrv_iostatus_disable(bs);

    notifier_list_init(&bs->close_notifiers);

    notifier_with_return_list_init(&bs->before_write_notifiers);

    qemu_co_queue_init(&bs->throttled_reqs);



    return bs;

}
@@ -1428,6 +1428,35 @@ void bdrv_close_all(void) 
    }

}



/* Check if any requests are in-flight (including throttled requests) */

static bool bdrv_requests_pending(BlockDriverState *bs)

{

    if (!QLIST_EMPTY(&bs->tracked_requests)) {

        return true;

    }

    if (!qemu_co_queue_empty(&bs->throttled_reqs)) {

        return true;

    }

    if (bs->file && bdrv_requests_pending(bs->file)) {

        return true;

    }

    if (bs->backing_hd && bdrv_requests_pending(bs->backing_hd)) {

        return true;

    }

    return false;

}



static bool bdrv_requests_pending_all(void)

{

    BlockDriverState *bs;

    QTAILQ_FOREACH(bs, &bdrv_states, list) {

        if (bdrv_requests_pending(bs)) {

            return true;

        }

    }

    return false;

}



/*

 * Wait for pending requests to complete across all BlockDriverStates

 *
@@ -1442,12 +1471,11 @@ void bdrv_close_all(void) 
 */

void bdrv_drain_all(void)

{

    /* Always run first iteration so any pending completion BHs run */

    bool busy = true;

    BlockDriverState *bs;

    bool busy;



    do {

        busy = qemu_aio_wait();



    while (busy) {

        /* FIXME: We do not have timer support here, so this is effectively

         * a busy wait.

         */
@@ -1456,12 +1484,9 @@ void bdrv_drain_all(void) 
                busy = true;

            }

        }

    } while (busy);



    /* If requests are still pending there is a bug somewhere */

    QTAILQ_FOREACH(bs, &bdrv_states, list) {

        assert(QLIST_EMPTY(&bs->tracked_requests));

        assert(qemu_co_queue_empty(&bs->throttled_reqs));

        busy = bdrv_requests_pending_all();

        busy |= aio_poll(qemu_get_aio_context(), busy);

    }

}

block/stream.c

+5 −1
Original line number Diff line number Diff line
@@ -57,6 +57,11 @@ static void close_unused_images(BlockDriverState *top, BlockDriverState *base, 
    BlockDriverState *intermediate;

    intermediate = top->backing_hd;



    /* Must assign before bdrv_delete() to prevent traversing dangling pointer

     * while we delete backing image instances.

     */

    top->backing_hd = base;



    while (intermediate) {

        BlockDriverState *unused;
@@ -70,7 +75,6 @@ static void close_unused_images(BlockDriverState *top, BlockDriverState *base, 
        unused->backing_hd = NULL;

        bdrv_delete(unused);

    }

    top->backing_hd = base;

}



static void coroutine_fn stream_run(void *opaque)