On Tue, Feb 08, 2022 at 09:35:10AM -0500, Emanuele Giuseppe Esposito wrote: > From: Paolo Bonzini <pbonz...@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 <pbonz...@redhat.com> > --- > job.c | 24 +++++++++++++++++++++--- > 1 file changed, 21 insertions(+), 3 deletions(-)
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
signature.asc
Description: PGP signature
