From: Paolo Bonzini Date: Mon, 26 Sep 2022 09:32:05 +0000 (-0400) Subject: job: detect change of aiocontext within job coroutine X-Git-Url: http://git.osdn.net/view?a=commitdiff_plain;h=ef02dac28e2ae7ac1d527dd715b459288bc652a5;p=qmiga%2Fqemu.git job: detect change of aiocontext within job coroutine 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. We want to avoid reading job->aio_context in job_enter_cond, therefore: 1) use aio_co_wake(), since it doesn't want an aiocontext as argument but uses job->co->ctx 2) detect possible discrepancy between job->co->ctx and job->aio_context by checking right after the coroutine resumes back from yielding if job->aio_context has changed. If so, reschedule the coroutine to the new context. 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() and 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: Vladimir Sementsov-Ogievskiy Reviewed-by: Stefan Hajnoczi Signed-off-by: Paolo Bonzini Message-Id: <20220926093214.506243-13-eesposit@redhat.com> Reviewed-by: Kevin Wolf Signed-off-by: Kevin Wolf --- diff --git a/job.c b/job.c index 926e385ac2..3ef5028751 100644 --- a/job.c +++ b/job.c @@ -588,7 +588,7 @@ void job_enter_cond_locked(Job *job, bool(*fn)(Job *job)) job->busy = true; real_job_unlock(); job_unlock(); - aio_co_enter(job->aio_context, job->co); + aio_co_wake(job->co); job_lock(); } @@ -615,6 +615,8 @@ 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); @@ -626,7 +628,20 @@ 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); }