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Date: Thu, 13 Sep 2018 18:59:54 +0200
From: Kevin Wolf <kwolf@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: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-block@nongnu.org, mreitz@redhat.com, famz@redhat.com, slp@redhat.com, jsnow@redhat.com, qemu-devel@nongnu.org

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?

Kevin