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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 17:51:21 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 28/11/2017 17:42, Jeff Cody wrote:
> On Tue, Nov 28, 2017 at 05:28:50PM +0100, Kevin Wolf wrote:
>> Am 28.11.2017 um 17:14 hat Paolo Bonzini geschrieben:
>>> 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.
>> Hm... With the reproducer we were specfically looking at
>> qmp_block_job_cancel(), which does take the AioContext locks. But it
>> might not be as universal as I thought.
>> To be honest, I just wasn't sure what to do with this case anyway. It
>> means that the coroutine is already running when someone else schedules
>> it. We don't really know whether we have to enter it a second time or
>> not.
>> So if it can indeed happen in practice, we need to think a bit more
>> about this.
> It would be nice if, on coroutine termination, we could unschedule all
> pending executions for that coroutine.  I think use-after-free is the main
> concern for someone else calling aio_co_schedule() while the coroutine is
> currently running.

Yes, terminating a scheduled coroutine is a bug; same for scheduling a
terminated coroutine, both orders are wrong. However, "unscheduling" is
not the solution; you would just be papering over the issue.

aio_co_schedule() on a running coroutine can only happen when the
coroutine is going to yield soon.


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