Re: [PATCH v6 15/18] job: detect change of aiocontext within job coroutine

[Emanuele Giuseppe Esposito]

Am 03/06/2022 um 18:59 schrieb Kevin Wolf:
> Am 14.03.2022 um 14:37 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. 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.
>>
>> 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: Stefan Hajnoczi <stefanha@redhat.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>  job.c | 24 +++++++++++++++++++++---
>>  1 file changed, 21 insertions(+), 3 deletions(-)
>>
>> diff --git a/job.c b/job.c
>> index 89c0e6bed9..10a5981748 100644
>> --- a/job.c
>> +++ b/job.c
>> @@ -543,11 +543,12 @@ void job_enter_cond_locked(Job *job, bool(*fn)(Job 
>> *job))
>>          return;
>>      }
>>  
>> -    assert(!job->deferred_to_main_loop);
> 
> Why doesn't this assertion hold true any more?

Theoretically this is useless, since we are in the same critical section
once the new lock is used, right? I don't recall any other reason, I
will restore it, if I need to respin (depends on what we decide in the
other patches feedback you provided)

Thank you,
Emanuele

> 
>>      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();
>>  }
>>  
>>  void job_enter(Job *job)
>> @@ -568,6 +569,8 @@ 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);
>> @@ -579,6 +582,20 @@ static void coroutine_fn job_do_yield_locked(Job *job, 
>> uint64_t ns)
>>      qemu_coroutine_yield();
>>      job_lock();
>>  
>> +    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;
>> +    }
>> +
>> +
> 
> Extra empty line.
> 
>>      /* Set by job_enter_cond_locked() before re-entering the coroutine.  */
>>      assert(job->busy);
>>  }
>> @@ -680,7 +697,6 @@ void job_resume_locked(Job *job)
>>      if (job->pause_count) {
>>          return;
>>      }
>> -
>>      /* kick only if no timer is pending */
>>      job_enter_cond_locked(job, job_timer_not_pending_locked);
>>  }
> 
> This hunk looks unrelated.
> 
> Kevin
>