| Message ID | 20220314133707.2206082-16-eesposit@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | job: replace AioContext lock with job_mutex | expand |
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? > 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
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 >
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); 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; + } + + /* 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); } @@ -1122,6 +1138,8 @@ 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);