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 9C551C433EF for ; Mon, 11 Jul 2022 14:56:17 +0000 (UTC) Received: from localhost ([::1]:36130 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1oAupc-0000d9-Nc for qemu-devel@archiver.kernel.org; Mon, 11 Jul 2022 10:56:16 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:43762) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1oAtwq-0000Ly-F7; Mon, 11 Jul 2022 09:59:41 -0400 Received: from forwardcorp1j.mail.yandex.net ([2a02:6b8:0:1619::183]:58260) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1oAtwl-00038g-JU; Mon, 11 Jul 2022 09:59:37 -0400 Received: from myt5-70c90f7d6d7d.qloud-c.yandex.net (myt5-70c90f7d6d7d.qloud-c.yandex.net [IPv6:2a02:6b8:c12:3e2c:0:640:70c9:f7d]) by forwardcorp1j.mail.yandex.net (Yandex) with ESMTP id 64D2E2E1B27; Mon, 11 Jul 2022 16:59:24 +0300 (MSK) Received: from [10.211.6.101] (10.211.6.101-vpn.dhcp.yndx.net [10.211.6.101]) by myt5-70c90f7d6d7d.qloud-c.yandex.net (smtpcorp/Yandex) with ESMTPSA id iyb3ld9OTF-xMOiETtu; Mon, 11 Jul 2022 16:59:23 +0300 (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)) (Client certificate not present) X-Yandex-Fwd: 1 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=yandex-team.ru; s=default; t=1657547963; bh=mdYNYp8lyaNFk4Hd1fDbtDJ8H7WuqPesZmKZ+HpqTJ0=; h=From:In-Reply-To:Cc:Date:References:To:Subject:Message-ID; b=jZZosqDh+zaKapu/x0sBdqO4DXm+UENRjjsrpcPmbQiDuG6Hh6MfROmwRfl6SxuE1 gQQB7Ut/CkAyzqrw/rtDkjTqTBfc7W5u1y/b+NjmRYurMy3EEIyh470BFc5NQJfDFl 20bBy+kTLCShcL656EJRNcsFJRbVD+f0CJiSTxgo= Authentication-Results: myt5-70c90f7d6d7d.qloud-c.yandex.net; dkim=pass header.i=@yandex-team.ru Message-ID: <402ade44-8309-aa69-682f-59560ddedd8d@yandex-team.ru> Date: Mon, 11 Jul 2022 16:59:22 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.8.1 Subject: Re: [PATCH v9 13/21] job: detect change of aiocontext within job coroutine Content-Language: en-US To: Emanuele Giuseppe Esposito , qemu-block@nongnu.org Cc: Kevin Wolf , Hanna Reitz , Paolo Bonzini , John Snow , Vladimir Sementsov-Ogievskiy , Wen Congyang , Xie Changlong , Markus Armbruster , Stefan Hajnoczi , Fam Zheng , qemu-devel@nongnu.org References: <20220706201533.289775-1-eesposit@redhat.com> <20220706201533.289775-14-eesposit@redhat.com> From: Vladimir Sementsov-Ogievskiy In-Reply-To: <20220706201533.289775-14-eesposit@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2a02:6b8:0:1619::183; envelope-from=vsementsov@yandex-team.ru; helo=forwardcorp1j.mail.yandex.net X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, 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" On 7/6/22 23:15, Emanuele Giuseppe Esposito wrote: > 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 Seems reasonable to me, still I don't have the whole picture in mind. Weak: Reviewed-by: Vladimir Sementsov-Ogievskiy > --- > job.c | 22 +++++++++++++++++++--- > 1 file changed, 19 insertions(+), 3 deletions(-) > > diff --git a/job.c b/job.c > index 9c8792c9e8..405f39566b 100644 > --- a/job.c > +++ b/job.c > @@ -581,11 +581,12 @@ void job_enter_cond_locked(Job *job, bool(*fn)(Job *job)) > return; > } > > - assert(!job->deferred_to_main_loop); why we remove this assertion? > 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_cond(Job *job, bool(*fn)(Job *job)) > @@ -611,6 +612,8 @@ 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); > @@ -622,7 +625,20 @@ 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); > } > -- Best regards, Vladimir