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Date: Fri, 14 Sep 2018 15:47:51 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight
 only after callback
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, qemu-block@nongnu.org, mreitz@redhat.com, slp@redhat.com, jsnow@redhat.com, qemu-devel@nongnu.org

On Thu, 09/13 18:59, Kevin Wolf wrote:
> Am 13.09.2018 um 17:10 hat Paolo Bonzini geschrieben:
> > On 13/09/2018 14:52, Kevin Wolf wrote:
> > > + if (qemu_get_current_aio_context() == qemu_get_aio_context()) {
> > > + /* If we are in the main thread, the callback is allowed to unref
> > > + * the BlockBackend, so we have to hold an additional reference */
> > > + blk_ref(acb->rwco.blk);
> > > + }
> > > acb->common.cb(acb->common.opaque, acb->rwco.ret);
> > > + blk_dec_in_flight(acb->rwco.blk);
> > > + if (qemu_get_current_aio_context() == qemu_get_aio_context()) {
> > > + blk_unref(acb->rwco.blk);
> > > + }
> > 
> > Is this something that happens only for some specific callers?  That is,
> > which callers are sure that the callback is invoked from the main thread?
> 
> I can't seem to reproduce the problem I saw any more even when reverting
> the bdrv_ref/unref pair. If I remember correctly it was actually a
> nested aio_poll() that was running a block job completion or something
> like that - which would obviously only happen on the main thread because
> the job intentionally defers to the main thread.
> 
> The only reason I made this conditional is that I think bdrv_unref()
> still isn't safe outside the main thread, is it?

I think that is correct.

Fam