Re: [PATCH v5 17/20] job: detect change of aiocontext within job coroutine

[Stefan Hajnoczi]

On Tue, Feb 08, 2022 at 09:35:10AM -0500, 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.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  job.c | 24 +++++++++++++++++++++---
>  1 file changed, 21 insertions(+), 3 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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