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Re: [Qemu-block] [Qemu-devel] [PATCH 2/3] block: use BDRV_POLL_WHILE() i

From: Stefan Hajnoczi
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 2/3] block: use BDRV_POLL_WHILE() in bdrv_rw_vmstate()
Date: Thu, 18 May 2017 09:06:24 +0100

On Thu, May 18, 2017 at 8:57 AM, Kevin Wolf <address@hidden> wrote:
> Am 17.05.2017 um 22:16 hat Eric Blake geschrieben:
>> On 05/17/2017 12:09 PM, Stefan Hajnoczi wrote:
>> > Calling aio_poll() directly may have been fine previously, but this is
>> > the future, man!
>> lol
>> >  The difference between an aio_poll() loop and
>> > BDRV_POLL_WHILE() is that BDRV_POLL_WHILE() releases the AioContext
>> > around aio_poll().
>> >
>> > This allows the IOThread to run fd handlers or BHs to complete the
>> > request.  Failure to release the AioContext causes deadlocks.
>> >
>> > Using BDRV_POLL_WHILE() partially fixes a 'savevm' hang with -object
>> > iothread.
>> I'm surprised at how many separate hangs we actually had!
> How hard would it be to write some test cases for this? Dataplane has
> a serious lack of automated testing.

And this hang doesn't even require in-flight guest I/O, so it would be
easy to reproduce in qemu-iotests if we add an IOThread mode.


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