Message ID | 20220208143513.1077229-18-eesposit@redhat.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | job: replace AioContext lock with job_mutex | expand |
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>
diff --git a/job.c b/job.c index 83cc9f52bd..f05850a337 100644 --- a/job.c +++ b/job.c @@ -529,11 +529,12 @@ void job_enter_cond_locked(Job *job, bool(*fn)(Job *job)) return; } - assert(!job->deferred_to_main_loop); 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) @@ -554,6 +555,8 @@ 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); @@ -565,6 +568,20 @@ 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; + } + + /* Set by job_enter_cond_locked() before re-entering the coroutine. */ assert(job->busy); } @@ -666,7 +683,6 @@ 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); } @@ -1117,6 +1133,8 @@ static void coroutine_fn job_co_entry(void *opaque) void job_start(Job *job) { + assert(qemu_in_main_thread()); + WITH_JOB_LOCK_GUARD() { assert(job && !job_started(job) && job->paused && job->driver && job->driver->run);