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author | Fam Zheng <famz@redhat.com> | 2016-04-05 19:20:52 +0800 |
---|---|---|
committer | Stefan Hajnoczi <stefanha@redhat.com> | 2016-04-11 16:59:09 +0100 |
commit | a77fd4bb2988c05953fdc9f1524085870ec1c939 (patch) | |
tree | 548c4dcfa3c1a56ac7670fea78198e830bf4afdd /block | |
parent | dc1ffa66619b3661f17a309b0aa8d65d8d29583f (diff) | |
download | qemu-a77fd4bb2988c05953fdc9f1524085870ec1c939.zip qemu-a77fd4bb2988c05953fdc9f1524085870ec1c939.tar.gz qemu-a77fd4bb2988c05953fdc9f1524085870ec1c939.tar.bz2 |
block: Fix bdrv_drain in coroutine
Using the nested aio_poll() in coroutine is a bad idea. This patch
replaces the aio_poll loop in bdrv_drain with a BH, if called in
coroutine.
For example, the bdrv_drain() in mirror.c can hang when a guest issued
request is pending on it in qemu_co_mutex_lock().
Mirror coroutine in this case has just finished a request, and the block
job is about to complete. It calls bdrv_drain() which waits for the
other coroutine to complete. The other coroutine is a scsi-disk request.
The deadlock happens when the latter is in turn pending on the former to
yield/terminate, in qemu_co_mutex_lock(). The state flow is as below
(assuming a qcow2 image):
mirror coroutine scsi-disk coroutine
-------------------------------------------------------------
do last write
qcow2:qemu_co_mutex_lock()
...
scsi disk read
tracked request begin
qcow2:qemu_co_mutex_lock.enter
qcow2:qemu_co_mutex_unlock()
bdrv_drain
while (has tracked request)
aio_poll()
In the scsi-disk coroutine, the qemu_co_mutex_lock() will never return
because the mirror coroutine is blocked in the aio_poll(blocking=true).
With this patch, the added qemu_coroutine_yield() allows the scsi-disk
coroutine to make progress as expected:
mirror coroutine scsi-disk coroutine
-------------------------------------------------------------
do last write
qcow2:qemu_co_mutex_lock()
...
scsi disk read
tracked request begin
qcow2:qemu_co_mutex_lock.enter
qcow2:qemu_co_mutex_unlock()
bdrv_drain.enter
> schedule BH
> qemu_coroutine_yield()
> qcow2:qemu_co_mutex_lock.return
> ...
tracked request end
...
(resumed from BH callback)
bdrv_drain.return
...
Reported-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1459855253-5378-2-git-send-email-famz@redhat.com
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Diffstat (limited to 'block')
-rw-r--r-- | block/io.c | 45 |
1 files changed, 45 insertions, 0 deletions
@@ -253,6 +253,47 @@ static void bdrv_drain_recurse(BlockDriverState *bs) } } +typedef struct { + Coroutine *co; + BlockDriverState *bs; + QEMUBH *bh; + bool done; +} BdrvCoDrainData; + +static void bdrv_co_drain_bh_cb(void *opaque) +{ + BdrvCoDrainData *data = opaque; + Coroutine *co = data->co; + + qemu_bh_delete(data->bh); + bdrv_drain(data->bs); + data->done = true; + qemu_coroutine_enter(co, NULL); +} + +void coroutine_fn bdrv_co_drain(BlockDriverState *bs) +{ + BdrvCoDrainData data; + + /* Calling bdrv_drain() from a BH ensures the current coroutine yields and + * other coroutines run if they were queued from + * qemu_co_queue_run_restart(). */ + + assert(qemu_in_coroutine()); + data = (BdrvCoDrainData) { + .co = qemu_coroutine_self(), + .bs = bs, + .done = false, + .bh = aio_bh_new(bdrv_get_aio_context(bs), bdrv_co_drain_bh_cb, &data), + }; + qemu_bh_schedule(data.bh); + + qemu_coroutine_yield(); + /* If we are resumed from some other event (such as an aio completion or a + * timer callback), it is a bug in the caller that should be fixed. */ + assert(data.done); +} + /* * Wait for pending requests to complete on a single BlockDriverState subtree, * and suspend block driver's internal I/O until next request arrives. @@ -269,6 +310,10 @@ void bdrv_drain(BlockDriverState *bs) bool busy = true; bdrv_drain_recurse(bs); + if (qemu_in_coroutine()) { + bdrv_co_drain(bs); + return; + } while (busy) { /* Keep iterating */ bdrv_flush_io_queue(bs); |