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Re: [Qemu-block] [PATCH v3 13/19] block: Remove aio_poll() in bdrv_drain

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH v3 13/19] block: Remove aio_poll() in bdrv_drain_poll variants
Date: Mon, 28 Jan 2019 21:13:15 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 20.09.18 18:19, Kevin Wolf wrote:
> bdrv_drain_poll_top_level() was buggy because it didn't release the
> AioContext lock of the node to be drained before calling aio_poll().
> This way, callbacks called by aio_poll() would possibly take the lock a
> second time and run into a deadlock with a nested AIO_WAIT_WHILE() call.
> However, it turns out that the aio_poll() call isn't actually needed any
> more. It was introduced in commit 91af091f923, which is effectively
> reverted by this patch. The cases it was supposed to fix are now covered
> by bdrv_drain_poll(), which waits for block jobs to reach a quiescent
> state.
> Signed-off-by: Kevin Wolf <address@hidden>
> Reviewed-by: Fam Zheng <address@hidden>
> Reviewed-by: Max Reitz <address@hidden>
> ---
>  block/io.c | 8 --------
>  1 file changed, 8 deletions(-)

Hm...  While looking at iotest 129 (which I think is broken because it
tries to use BB-level throttling which doesn't do anything for the
mirror job), I noticed this:

$ x86_64-softmmu/qemu-system-x86_64 \
    -object throttle-group,id=tg0 \
    -drive node-name=node0,driver=throttle,\
file.file.filename=/tmp/src.qcow2 -qmp stdio \

qemu-system-x86_64: block/block-backend.c:2211: blk_root_drained_end:
Assertion `blk->quiesce_counter' failed.
[1]    2722 abort (core dumped)  x86_64-softmmu/qemu-system-x86_64
-object throttle-group,id=tg0 -drive  -qmp

(Which worked before commit 4cf077b59fc73eec29f8b7d082919dbb278bdc86,
i.e. this one.)


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