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 <vsementsov@yandex-team.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220926093214.506243-13-eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>