Re: [Qemu-devel] [Qemu-block] [PATCH] mirror: hold aio_context before bdrv_drain

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Apr 01, 2015 at 04:49:39PM +0800, Bin Wu wrote:
> 
> On 2015/4/1 16:19, Fam Zheng wrote:
> > On Wed, 04/01 12:42, Bin Wu wrote:
> >> From: Bin Wu <wu.wubin@huawei.com>
> > 
> > What's the issue are you fixing? I think the coroutine already is running in
> > the AioContext of bs.
> > 
> > Fam
> > 
> In the current implementation of bdrv_drain, it should be placed in a critical
> section as suggested in the comments above the function: "Note that unlike
> bdrv_drain_all(), the caller must hold the BlockDriverState AioContext".
> 
> However, the mirror coroutine starting with mirror_run doesn't do this. I just
> found qmp_drive_mirror protects the AioCentext, but it is out of the scope of
> the mirror coroutine.

There are 3 possibilities:

1. qmp_drive_mirror() under QEMU main loop thread.  AioContext is held.

2. IOThread aio_poll().  AioContext is held.

3. QEMU main loop thread when IOThread (dataplane) is not used.  Here
   the AioContext is the global qemu_aio_context.  We don't need to
   acquire it explicitly, we're protected by the global mutex.

If #3 was a problem then virtio-blk.c's bdrv_aio_writev() would also be
a problem, for example.

This patch is unnecessary.

Stefan

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