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From: Stefan Hajnoczi <stefanha@gmail.com>
Date: Thu, 18 May 2017 09:05:09 +0100
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Subject: Re: [Qemu-devel] [Qemu-block] [PATCH 2/3] block: use
 BDRV_POLL_WHILE() in bdrv_rw_vmstate()
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To: Eric Blake <eblake@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Kevin Wolf <kwolf@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Fam Zheng <famz@redhat.com>, qemu block <qemu-block@nongnu.org>

On Wed, May 17, 2017 at 9:16 PM, Eric Blake <eblake@redhat.com> wrote:
> On 05/17/2017 12:09 PM, Stefan Hajnoczi wrote:
>> diff --git a/block/io.c b/block/io.c
>> index cc56e90..f0041cd 100644
>> --- a/block/io.c
>> +++ b/block/io.c
>> @@ -2031,9 +2031,7 @@ bdrv_rw_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos,
>>          Coroutine *co = qemu_coroutine_create(bdrv_co_rw_vmstate_entry, &data);
>>
>>          bdrv_coroutine_enter(bs, co);
>> -        while (data.ret == -EINPROGRESS) {
>> -            aio_poll(bdrv_get_aio_context(bs), true);
>> -        }
>> +        BDRV_POLL_WHILE(bs, data.ret == -EINPROGRESS);
>>          return data.ret;
>>      }
>
> Do we have other culprits (not necessarily for vmsave, but for other
> situations), where we should be using BDRV_POLL_WHILE in separate
> patches? For example, a quick grep show that at least hw/9pfs/9p.c makes
> a direct call to aio_poll(,true) in a while loop.

virtio-9p doesn't use IOThread (dataplane) so it is not affected.  It
also doesn't use BlockDriverState so it cannot use BDRV_POLL_WHILE().

I did grep for other aio_poll() callers and didn't spot any obvious
cases that need to be fixed.  They tend to run in situations where
this deadlock cannot occur.

Stefan