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Date: Thu, 26 Mar 2015 11:45:02 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20150326034502.GF14724@ad.nay.redhat.com>
References: <20150325083139.GB14724@ad.nay.redhat.com>
	<20150325133113.GK22940@stefanha-thinkpad.redhat.com>
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In-Reply-To: <20150325133113.GK22940@stefanha-thinkpad.redhat.com>
Subject: Re: [Qemu-devel] AioContext of block jobs
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To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: pbonzini@redhat.com, qemu-devel@nongnu.org

On Wed, 03/25 13:31, Stefan Hajnoczi wrote:
> On Wed, Mar 25, 2015 at 04:31:39PM +0800, Fam Zheng wrote:
> > I was looking at block jobs' AioContext and realized that the block job
> > coroutines are actually started in main loop.
> > 
> > I'm confused because 5a7e7a0bad17c96e03f55ed7019e2d7545e21a96 and friends in
> > the series [1] seem to move the coroutines to the BDS's iothreads, but it
> > didn't do that.
> > 
> > (Although after the first block_job_yield or sleep, the coroutines ARE resumed
> > in the right AioContext.)
> > 
> > Why is it safe to start the jobs from the main thread where QMP command is
> > handled? I see no guarantee that the jobs won't access BDS before first yield
> > but after releasing the AioContext.
> 
> Is there a concrete case you are worried about?

No, I just missed that aio_context_release is only called after the first block
job coroutine yield.

Thanks for explaining.

Fam

> 
> For example:
> void qmp_block_stream(const char *device,
> ...
>     stream_start(bs, base_bs, base_name, has_speed ? speed : 0,
>                  on_error, block_job_cb, bs, &local_err);
>     if (local_err) {
>         error_propagate(errp, local_err);
>         goto out;
>     }
> 
>     trace_qmp_block_stream(bs, bs->job);
> 
> out:
>     aio_context_release(aio_context);   <----
> }
> 
> Since the BDS AioContext is held during stream_start, there is no race
> condition.
> 
> The resources used by the coroutine (i.e. timers or the BDS) bind their event
> handler functions to the AioContext.  This means the coroutine will only be
> entered again under the AioContext in the future.
> 
> Stefan