Re: [PATCH v10 13/21] job: detect change of aiocontext within job coroutine

[Emanuele Giuseppe Esposito]

Am 05/08/2022 um 10:37 schrieb Kevin Wolf:
> Am 25.07.2022 um 09:38 hat Emanuele Giuseppe Esposito geschrieben:
>> From: Paolo Bonzini <pbonzini@redhat.com>
>>
>> We want to make sure access of job->aio_context is always done
>> under either BQL or job_mutex.
> 
> Is this the goal of this series? If so, it would have been useful to
> state somewhere more obvious, because I had assumed that holding the BQL
> would not be considered enough, but everyone needs to hold the job_mutex.

It is the goal for this patch :)
The whole job API can't rely on BQL since there are coroutines running
in another aiocontext.
> 
>> The problem is that using
>> aio_co_enter(job->aiocontext, job->co) in job_start and job_enter_cond
>> makes the coroutine immediately resume, so we can't hold the job lock.
>> And caching it is not safe either, as it might change.
>>
>> job_start is under BQL, so it can freely read job->aiocontext, but
>> job_enter_cond is not. In order to fix this, use aio_co_wake():
>> the advantage is that it won't use job->aiocontext, but the
>> main disadvantage is that it won't be able to detect a change of
>> job AioContext.
>>
>> Calling bdrv_try_set_aio_context() will issue the following calls
>> (simplified):
>> * in terms of  bdrv callbacks:
>>   .drained_begin -> .set_aio_context -> .drained_end
>> * in terms of child_job functions:
>>   child_job_drained_begin -> child_job_set_aio_context -> 
>> child_job_drained_end
>> * in terms of job functions:
>>   job_pause_locked -> job_set_aio_context -> job_resume_locked
>>
>> We can see that after setting the new aio_context, job_resume_locked
>> calls again job_enter_cond, which then invokes aio_co_wake(). But
>> while job->aiocontext has been set in job_set_aio_context,
>> job->co->ctx has not changed, so the coroutine would be entering in
>> the wrong aiocontext.
>>
>> Using aio_co_schedule in job_resume_locked() might seem as a valid
>> alternative, but the problem is that the bh resuming the coroutine
>> is not scheduled immediately, and if in the meanwhile another
>> bdrv_try_set_aio_context() is run (see test_propagate_mirror() in
>> test-block-iothread.c), we would have the first schedule in the
>> wrong aiocontext, and the second set of drains won't even manage
>> to schedule the coroutine, as job->busy would still be true from
>> the previous job_resume_locked().
>>
>> The solution is to stick with aio_co_wake(), but then detect every time
>> the coroutine resumes back from yielding if job->aio_context
>> has changed. If so, we can reschedule it to the new context.
> 
> Hm, but with this in place, what does aio_co_wake() actually buy us
> compared to aio_co_enter()?
> 
> I guess it's a bit simpler code because you don't have to explicitly
> specify the AioContext, but we're still going to enter the coroutine in
> the wrong AioContext occasionally and have to reschedule it, just like
> in the existing code (except that the rescheduling doesn't exist there
> yet).
> 
> So while I don't disagree with the change, I don't think the
> justification in the commit message is right for this part.

What do you suggest to change?

> 
>> Check for the aiocontext change in job_do_yield_locked because:
>> 1) aio_co_reschedule_self requires to be in the running coroutine
>> 2) since child_job_set_aio_context allows changing the aiocontext only
>>    while the job is paused, this is the exact place where the coroutine
>>    resumes, before running JobDriver's code.
>>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
>> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>  job.c | 21 +++++++++++++++++++--
>>  1 file changed, 19 insertions(+), 2 deletions(-)
>>
>> diff --git a/job.c b/job.c
>> index b0729e2bb2..ecec66b44e 100644
>> --- a/job.c
>> +++ b/job.c
>> @@ -585,7 +585,9 @@ void job_enter_cond_locked(Job *job, bool(*fn)(Job *job))
>>      timer_del(&job->sleep_timer);
>>      job->busy = true;
>>      real_job_unlock();
>> -    aio_co_enter(job->aio_context, job->co);
>> +    job_unlock();
>> +    aio_co_wake(job->co);
>> +    job_lock();
> 
> The addition of job_unlock/lock is unrelated, this was necessary even
> before this patch.

Ok

> 
>>  }
>>  
>>  void job_enter_cond(Job *job, bool(*fn)(Job *job))
>> @@ -611,6 +613,8 @@ void job_enter(Job *job)
>>   */
>>  static void coroutine_fn job_do_yield_locked(Job *job, uint64_t ns)
>>  {
>> +    AioContext *next_aio_context;
>> +
>>      real_job_lock();
>>      if (ns != -1) {
>>          timer_mod(&job->sleep_timer, ns);
>> @@ -622,7 +626,20 @@ static void coroutine_fn job_do_yield_locked(Job *job, 
>> uint64_t ns)
>>      qemu_coroutine_yield();
>>      job_lock();
>>  
>> -    /* Set by job_enter_cond() before re-entering the coroutine.  */
>> +    next_aio_context = job->aio_context;
>> +    /*
>> +     * Coroutine has resumed, but in the meanwhile the job AioContext
>> +     * might have changed via bdrv_try_set_aio_context(), so we need to move
>> +     * the coroutine too in the new aiocontext.
>> +     */
>> +    while (qemu_get_current_aio_context() != next_aio_context) {
>> +        job_unlock();
>> +        aio_co_reschedule_self(next_aio_context);
>> +        job_lock();
>> +        next_aio_context = job->aio_context;
>> +    }
>> +
>> +    /* Set by job_enter_cond_locked() before re-entering the coroutine.  */
>>      assert(job->busy);
>>  }
> 

Emanuele