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Re: [Qemu-devel] [PATCH for-2.11 3/4] coroutine: Cancel aio_co_schedule(

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH for-2.11 3/4] coroutine: Cancel aio_co_schedule() on direct entry
Date: Tue, 28 Nov 2017 17:14:16 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 28/11/2017 16:43, Kevin Wolf wrote:
> +    /* Make sure that a coroutine that can alternatively reentered from two
> +     * different sources isn't reentered more than once when the first caller
> +     * uses aio_co_schedule() and the other one enters to coroutine directly.
> +     * This is achieved by cancelling the pending aio_co_schedule().
> +     *
> +     * The other way round, if aio_co_schedule() would be called after this
> +     * point, this would be a problem, too, but in practice it doesn't happen
> +     * because we're holding the AioContext lock here and aio_co_schedule()
> +     * callers must do the same.

No, this is not true.  aio_co_schedule is thread-safe.

> This means that the coroutine just needs to
> +     * prevent other callers from calling aio_co_schedule() before it yields
> +     * (e.g. block job coroutines by setting job->busy = true).
> +     *
> +     * We still want to ensure that the second case doesn't happen, so reset
> +     * co->scheduled only after setting co->caller to make the above check
> +     * effective for the co_schedule_bh_cb() case. */
> +    atomic_set(&co->scheduled, NULL);

This doesn't work.  The coroutine is still in the list, and if someone
calls aio_co_schedule again now, any coroutines linked from "co" via
co_scheduled_next are lost.

(Also, the AioContext lock is by design not protecting any state in
AioContext itself; the AioContext lock is only protecting things that
run in an AioContext but do not have their own lock).


> +
>      ret = qemu_coroutine_switch(self, co, COROUTINE_ENTER);
>      qemu_co_queue_run_restart(co);

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