Re: [Qemu-devel] [PATCH v3 2/2] virtio-blk: Use blk_drain() to drain IO requests

[Stefan Hajnoczi]

On Mon, Jun 29, 2015 at 08:10:20AM +0200, Markus Armbruster wrote:
> Alexander Yarygin <address@hidden> writes:
> > Markus Armbruster <address@hidden> writes:
> > * Ignorant answer: I was told that the bdrv_drain_all()'s comment is
> >   obsolete and we can use bdrv_drain(). Here is a link to the old
> >   thread: http://marc.info/?l=qemu-devel&m=143154211017926&w=2.
> 
> Kevin, Stefan, if the comment has become wrong, it needs to be redone.
> Who's going to take care of it?

I couldn't think of a scenario where this patch is unsafe.

The danger is that the I/O code path depends on something outside the
AioContext.  In that case you block in aio_poll(aio_context, true)
forever without making progress.  The thing the I/O request depends on
will never finish.

Code that accesses multiple BDSes puts them into the same AioContext, so
this should not be a problem in practice.

Stefan

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