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Re: [PATCH v10 13/21] job: detect change of aiocontext within job corout

From: Emanuele Giuseppe Esposito
Subject: Re: [PATCH v10 13/21] job: detect change of aiocontext within job coroutine
Date: Wed, 17 Aug 2022 13:16:44 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0

Am 17/08/2022 um 10:34 schrieb Kevin Wolf:
> Am 16.08.2022 um 17:09 hat Emanuele Giuseppe Esposito geschrieben:
>> Am 05/08/2022 um 10:37 schrieb Kevin Wolf:
>>> Am 25.07.2022 um 09:38 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.
>>> Is this the goal of this series? If so, it would have been useful to
>>> state somewhere more obvious, because I had assumed that holding the BQL
>>> would not be considered enough, but everyone needs to hold the job_mutex.
>> It is the goal for this patch :)
>> The whole job API can't rely on BQL since there are coroutines running
>> in another aiocontext.
> Yes, as I saw in patch 14, which describes the goal more clearly in the
> commit message and also adds the corresponding documentation to
> Job.aio_context. Maybe it would have been clearer if the documentation
> were already in this patch.

I don't understand what you mean here, sorry. You mean
job_set_aiocontext documentation? I don't see what is confusing here,
could you please clarify?

>>>> 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.
>>> Hm, but with this in place, what does aio_co_wake() actually buy us
>>> compared to aio_co_enter()?
>>> I guess it's a bit simpler code because you don't have to explicitly
>>> specify the AioContext, but we're still going to enter the coroutine in
>>> the wrong AioContext occasionally and have to reschedule it, just like
>>> in the existing code (except that the rescheduling doesn't exist there
>>> yet).
>>> So while I don't disagree with the change, I don't think the
>>> justification in the commit message is right for this part.
>> What do you suggest to change?
> The commit message shouldn't pretend that aio_co_wake() solves the
> problem (it says "In order to fix this, use aio_co_wake"), even if
> that's what you thought at first before you saw that the problem wasn't
> fully fixed by it.
> I would move the real solution up in the commit message ("In order to
> fix this, detect every time..."), and then maybe mention why
> aio_co_wake() doesn't solve the problem, but you're leaving it in anyway
> because it's nicer than the previous sequence or something like that.

I think the issue is "In order to fix this", meaning what "this" refers
to. It is not the fix, it is the problem stated in the previous sentence.

Since job_enter_cond is not under BQL, we can't read job->aiocontext,
therefore use aio_co_wake to avoid doing that.

Would it be ok if I replace the paragraph with:

job_start is under BQL, so it can freely read job->aiocontext, but
job_enter_cond is not. In order to avoid reading the job aiocontext, use
aio_co_wake(), since it resorts so job->co->ctx. The
only disadvantage is that it won't be able to detect a change of
job AioContext, as explained below.

>>>> 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>
> Kevin

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