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 X-Spam-Level: X-Spam-Status: No, score=-15.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AC73EC433ED for ; Thu, 8 Apr 2021 16:57:54 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 47933610CB for ; Thu, 8 Apr 2021 16:57:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 47933610CB Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:49946 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lUXyb-0006XP-Aj for qemu-devel@archiver.kernel.org; Thu, 08 Apr 2021 12:57:53 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:54118) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lUXwo-0005Ra-1J for qemu-devel@nongnu.org; Thu, 08 Apr 2021 12:56:02 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:38083) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lUXwk-00075O-6I for qemu-devel@nongnu.org; Thu, 08 Apr 2021 12:56:01 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617900952; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=h+P+ZffSgIgVdAI5r9UUDvNDUJl7zc2QxFY0uXcs0JU=; b=Dp8+5cVtvKkKSG/St2WH+DlSMOHh3FuHQuZP3YStt2NdqWoVQ1QfoWzyD7em7dlUTmYkw2 A7/Tg9Z/iR8j7DEK6fUhCZdc8grvn3ROQ3rVL6z4+9xYWTDCId2JJfUDIE4IdIu0OTT3EL UVLua8WwJh8Dx51a3ETKK84dY2o8/Bw= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-550-t2CKaYpjNf6QWUeJGfOXjA-1; Thu, 08 Apr 2021 12:55:50 -0400 X-MC-Unique: t2CKaYpjNf6QWUeJGfOXjA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 35DA18070FF; Thu, 8 Apr 2021 16:55:49 +0000 (UTC) Received: from [10.10.117.61] (ovpn-117-61.rdu2.redhat.com [10.10.117.61]) by smtp.corp.redhat.com (Postfix) with ESMTP id 7CBD02C167; Thu, 8 Apr 2021 16:55:48 +0000 (UTC) Subject: Re: [PATCH for-6.0? 1/3] job: Add job_wait_unpaused() for block-job-complete To: Max Reitz , qemu-block@nongnu.org References: <20210408162039.242670-1-mreitz@redhat.com> <20210408162039.242670-2-mreitz@redhat.com> From: John Snow Message-ID: Date: Thu, 8 Apr 2021 12:55:47 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210408162039.242670-2-mreitz@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jsnow@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=jsnow@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 4/8/21 12:20 PM, Max Reitz wrote: > block-job-complete can only be applied when the job is READY, not when > it is on STANDBY (ready, but paused). Draining a job technically pauses > it (which makes a READY job enter STANDBY), and ending the drained > section does not synchronously resume it, but only schedules the job, > which will then be resumed. So attempting to complete a job immediately > after a drained section may sometimes fail. > > That is bad at least because users cannot really work nicely around > this: A job may be paused and resumed at any time, so waiting for the > job to be in the READY state and then issuing a block-job-complete poses > a TOCTTOU problem. The only way around it would be to issue > block-job-complete until it no longer fails due to the job being in the > STANDBY state, but that would not be nice. > > We can solve the problem by allowing block-job-complete to be invoked on > jobs that are on STANDBY, if that status is the result of a drained > section (not because the user has paused the job), and that section has > ended. That is, if the job is on STANDBY, but scheduled to be resumed. > > Perhaps we could actually just directly allow this, seeing that mirror > is the only user of ready/complete, and that mirror_complete() could > probably work under the given circumstances, but there may be many side > effects to consider. > > It is simpler to add a function job_wait_unpaused() that waits for the > job to be resumed (under said circumstances), and to make > qmp_block_job_complete() use it to delay job_complete() until then. > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1945635 > Signed-off-by: Max Reitz > --- > include/qemu/job.h | 15 +++++++++++++++ > blockdev.c | 3 +++ > job.c | 42 ++++++++++++++++++++++++++++++++++++++++++ > 3 files changed, 60 insertions(+) > > diff --git a/include/qemu/job.h b/include/qemu/job.h > index efc6fa7544..cf3082b6d7 100644 > --- a/include/qemu/job.h > +++ b/include/qemu/job.h > @@ -563,4 +563,19 @@ void job_dismiss(Job **job, Error **errp); > */ > int job_finish_sync(Job *job, void (*finish)(Job *, Error **errp), Error **errp); > > +/** > + * If the job has been paused because of a drained section, and that > + * section has ended, wait until the job is resumed. > + * > + * Return 0 if the job is not paused, or if it has been successfully > + * resumed. > + * Return an error if the job has been paused in such a way that > + * waiting will not resume it, i.e. if it has been paused by the user, > + * or if it is still drained. > + * > + * Callers must be in the home AioContext and hold the AioContext lock > + * of job->aio_context. > + */ > +int job_wait_unpaused(Job *job, Error **errp); > + > #endif > diff --git a/blockdev.c b/blockdev.c > index a57590aae4..c0cc2fa364 100644 > --- a/blockdev.c > +++ b/blockdev.c > @@ -3414,6 +3414,9 @@ void qmp_block_job_complete(const char *device, Error **errp) > return; > } > > + if (job_wait_unpaused(&job->job, errp) < 0) { > + return; > + } After which point, we assume we've transitioned back to either RUNNING or READY, and > trace_qmp_block_job_complete(job); > job_complete(&job->job, errp); This function checks the usual state table for permission to deliver/perform the verb. > aio_context_release(aio_context); > diff --git a/job.c b/job.c > index 289edee143..1ea30fd294 100644 > --- a/job.c > +++ b/job.c > @@ -1023,3 +1023,45 @@ int job_finish_sync(Job *job, void (*finish)(Job *, Error **errp), Error **errp) > job_unref(job); > return ret; > } > + > +int job_wait_unpaused(Job *job, Error **errp) > +{ > + /* > + * Only run this function from the main context, because this is > + * what we need, and this way we do not have to think about what > + * happens if the user concurrently pauses the job from the main > + * monitor. > + */ > + assert(qemu_get_current_aio_context() == qemu_get_aio_context()); > + > + /* > + * Quick path (e.g. so we do not get an error if pause_count > 0 > + * but the job is not even paused) > + */ > + if (!job->paused) { > + return 0; > + } > + > + /* If the user has paused the job, waiting will not help */ > + if (job->user_paused) { > + error_setg(errp, "Job '%s' has been paused by the user", job->id); > + return -EBUSY; > + } > + Or the job has encountered an error if that error policy is set. It is maybe more accurate to say that the job is currently paused/halted (for some reason) and is awaiting the explicit unpause instruction. "Job '%s' has been paused and needs to be explicitly resumed with job-resume", maybe? Job '%s' has been paused and needs to be [explicitly] resumed [by the user] [with job-resume] Some combo of those runes. > + /* Similarly, if the job is still drained, waiting will not help either */ > + if (job->pause_count > 0) { > + error_setg(errp, "Job '%s' is blocked and cannot be unpaused", job->id); > + return -EBUSY; > + } > + This leaks an internal state detail out to the caller. In which circumstances does this happen? Do we expect it to? As the user: Why is it blocked? Can I unblock it? Do I wait? > + /* > + * This function is specifically for waiting for a job to be > + * resumed after a drained section. Ending the drained section > + * includes a job_enter(), which schedules the job loop to be run, > + * and once it does, job->paused will be cleared. Therefore, we > + * do not need to invoke job_enter() here. > + */ > + AIO_WAIT_WHILE(job->aio_context, job->paused); > + > + return 0; > +} > Looks about right to me, but you'll want Kevin's look-see for the finer details, of course. My concern is that this adds a wait of an indefinite period to the job_complete command. We mitigate this by checking for some other internal state criteria first, and then by process of elimination deduce that it's safe to wait, as it will (likely) be very quick. Do we open the door for ourselves to get into trouble here, either by a state we are forgetting to rule out (You'd have added it if you know the answer to this) or a hypothetical future change where we forget to update this function? Not necessarily a blocker, I think, and this does solve a real problem fairly inexpensively. On good faith that you understand the synchronicity issues here better than I do: Reviewed-by: John Snow