Re: [Qemu-devel] AioContext of block jobs

[Stefan Hajnoczi]

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?

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

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