From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 40F4CC433EF for ; Fri, 3 Jun 2022 17:01:28 +0000 (UTC) Received: from localhost ([::1]:45480 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nxAfu-0003PX-I8 for qemu-devel@archiver.kernel.org; Fri, 03 Jun 2022 13:01:26 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:56912) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nxAdu-0002So-Eu for qemu-devel@nongnu.org; Fri, 03 Jun 2022 12:59:22 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:44526) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nxAdr-0004Tr-E7 for qemu-devel@nongnu.org; Fri, 03 Jun 2022 12:59:21 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1654275551; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=DbiBVAyrHxAxK/QEy0ZwkTY8yyfM70jxWpQ1jGpyrC8=; b=RJC/29FDAwJBV73yfIfdWHwD9kHoj8P8GqwX4ndvnmlCinpHl922PoQRFAHFaCrYuMiFgW ZwisQPyhWodv0rUC1yBUK65GvxElubD8hUbaPT9iGc9I0q+B0r5q2/tcwFFKSs8LP145yT XRH9lyBcw/mLkLkx83tl8RIVBtHvlmY= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-453-mt114-A-NLejjcOU_j60fg-1; Fri, 03 Jun 2022 12:59:07 -0400 X-MC-Unique: mt114-A-NLejjcOU_j60fg-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 4B046185A7B2; Fri, 3 Jun 2022 16:59:07 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.36]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1D47F2026D07; Fri, 3 Jun 2022 16:59:05 +0000 (UTC) Date: Fri, 3 Jun 2022 18:59:03 +0200 From: Kevin Wolf To: Emanuele Giuseppe Esposito Cc: qemu-block@nongnu.org, Hanna Reitz , Paolo Bonzini , John Snow , Vladimir Sementsov-Ogievskiy , Wen Congyang , Xie Changlong , Markus Armbruster , Stefan Hajnoczi , Fam Zheng , qemu-devel@nongnu.org Subject: Re: [PATCH v6 15/18] job: detect change of aiocontext within job coroutine Message-ID: References: <20220314133707.2206082-1-eesposit@redhat.com> <20220314133707.2206082-16-eesposit@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220314133707.2206082-16-eesposit@redhat.com> X-Scanned-By: MIMEDefang 2.78 on 10.11.54.4 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 14.03.2022 um 14:37 hat Emanuele Giuseppe Esposito geschrieben: > From: Paolo Bonzini > > 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 > Signed-off-by: Paolo Bonzini > --- > 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