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[Qemu-devel] [PULL 34/42] block-backend: Decrease in_flight only after c
From: |
Max Reitz |
Subject: |
[Qemu-devel] [PULL 34/42] block-backend: Decrease in_flight only after callback |
Date: |
Tue, 25 Sep 2018 17:15:33 +0200 |
From: Kevin Wolf <address@hidden>
Request callbacks can do pretty much anything, including operations that
will yield from the coroutine (such as draining the backend). In that
case, a decreased in_flight would be visible to other code and could
lead to a drain completing while the callback hasn't actually completed
yet.
Note that reordering these operations forbids calling drain directly
inside an AIO callback. As Paolo explains, indirectly calling it is
okay:
- Calling it through a coroutine is okay, because then
bdrv_drained_begin() goes through bdrv_co_yield_to_drain() and you
have in_flight=2 when bdrv_co_yield_to_drain() yields, then soon
in_flight=1 when the aio_co_wake() in the AIO callback completes, then
in_flight=0 after the bottom half starts.
- Calling it through a bottom half would be okay too, as long as the AIO
callback remembers to do inc_in_flight/dec_in_flight just like
bdrv_co_yield_to_drain() and bdrv_co_drain_bh_cb() do
A few more important cases that come to mind:
- A coroutine that yields because of I/O is okay, with a sequence
similar to bdrv_co_yield_to_drain().
- A coroutine that yields with no I/O pending will correctly decrease
in_flight to zero before yielding.
- Calling more AIO from the callback won't overflow the counter just
because of mutual recursion, because AIO functions always yield at
least once before invoking the callback.
Signed-off-by: Kevin Wolf <address@hidden>
Reviewed-by: Fam Zheng <address@hidden>
Reviewed-by: Max Reitz <address@hidden>
Reviewed-by: Paolo Bonzini <address@hidden>
---
block/block-backend.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/block/block-backend.c b/block/block-backend.c
index f71fdb4315..551e4e1708 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1339,8 +1339,8 @@ static const AIOCBInfo blk_aio_em_aiocb_info = {
static void blk_aio_complete(BlkAioEmAIOCB *acb)
{
if (acb->has_returned) {
- blk_dec_in_flight(acb->rwco.blk);
acb->common.cb(acb->common.opaque, acb->rwco.ret);
+ blk_dec_in_flight(acb->rwco.blk);
qemu_aio_unref(acb);
}
}
--
2.17.1
- [Qemu-devel] [PULL 24/42] job: Fix missing locking due to mismerge, (continued)
- [Qemu-devel] [PULL 24/42] job: Fix missing locking due to mismerge, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 25/42] blockjob: Wake up BDS when job becomes idle, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 26/42] aio-wait: Increase num_waiters even in home thread, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 23/42] job: Fix nested aio_poll() hanging in job_txn_apply, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 28/42] test-blockjob: Acquire AioContext around job_cancel_sync(), Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 27/42] test-bdrv-drain: Drain with block jobs in an I/O thread, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 30/42] test-bdrv-drain: Test AIO_WAIT_WHILE() in completion callback, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 29/42] job: Use AIO_WAIT_WHILE() in job_finish_sync(), Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 31/42] block: Add missing locking in bdrv_co_drain_bh_cb(), Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 32/42] block-backend: Add .drained_poll callback, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 34/42] block-backend: Decrease in_flight only after callback,
Max Reitz <=
- [Qemu-devel] [PULL 33/42] block-backend: Fix potential double blk_delete(), Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 35/42] blockjob: Lie better in child_job_drained_poll(), Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 36/42] block: Remove aio_poll() in bdrv_drain_poll variants, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 38/42] job: Avoid deadlocks in job_completed_txn_abort(), Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 37/42] test-bdrv-drain: Test nested poll in bdrv_drain_poll_top_level(), Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 40/42] test-bdrv-drain: Fix outdated comments, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 39/42] test-bdrv-drain: AIO_WAIT_WHILE() in job .commit/.abort, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 42/42] test-bdrv-drain: Test draining job source child and parent, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 41/42] block: Use a single global AioWait, Max Reitz, 2018/09/25
- [Qemu-devel] [PULL 04/42] block/commit: refactor commit to use job callbacks, Max Reitz, 2018/09/25