[v9,13/21] job: detect change of aiocontext within job coroutine

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 | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

On 7/6/22 23:15, Emanuele Giuseppe Esposito wrote:
> 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>

Seems reasonable to me, still I don't have the whole picture in mind. Weak:

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

> ---
>   job.c | 22 +++++++++++++++++++---
>   1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/job.c b/job.c
> index 9c8792c9e8..405f39566b 100644
> --- a/job.c
> +++ b/job.c
> @@ -581,11 +581,12 @@ void job_enter_cond_locked(Job *job, bool(*fn)(Job *job))
>           return;
>       }
>   
> -    assert(!job->deferred_to_main_loop);

why we remove this assertion?

>       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_cond(Job *job, bool(*fn)(Job *job))
> @@ -611,6 +612,8 @@ 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 +625,20 @@ 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);
>   }
>