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 us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 C0B67C433F5 for ; Tue, 14 Dec 2021 10:11:56 +0000 (UTC) Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-474-npvB7mO7NC6Sgg3rr_5WKA-1; Tue, 14 Dec 2021 05:11:48 -0500 X-MC-Unique: npvB7mO7NC6Sgg3rr_5WKA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id ADC821023F4E; Tue, 14 Dec 2021 10:11:44 +0000 (UTC) Received: from colo-mx.corp.redhat.com (colo-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.20]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 472B14ABAD; Tue, 14 Dec 2021 10:11:43 +0000 (UTC) Received: from lists01.pubmisc.prod.ext.phx2.redhat.com (lists01.pubmisc.prod.ext.phx2.redhat.com [10.5.19.33]) by colo-mx.corp.redhat.com (Postfix) with ESMTP id 97C781809CB8; Tue, 14 Dec 2021 10:11:41 +0000 (UTC) Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) by lists01.pubmisc.prod.ext.phx2.redhat.com (8.13.8/8.13.8) with ESMTP id 1BEABdCj017166 for ; Tue, 14 Dec 2021 05:11:39 -0500 Received: by smtp.corp.redhat.com (Postfix) id 1A73C2166B26; Tue, 14 Dec 2021 10:11:39 +0000 (UTC) Received: from mimecast-mx02.redhat.com (mimecast08.extmail.prod.ext.rdu2.redhat.com [10.11.55.24]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 149C82166B25 for ; Tue, 14 Dec 2021 10:11:36 +0000 (UTC) Received: from us-smtp-1.mimecast.com (us-smtp-delivery-1.mimecast.com [207.211.31.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 0521E3800E92 for ; Tue, 14 Dec 2021 10:11:36 +0000 (UTC) Received: from out1.migadu.com (out1.migadu.com [91.121.223.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-131-3Gg_KTgSMAaM6l09eDDogg-1; Tue, 14 Dec 2021 05:11:32 -0500 X-MC-Unique: 3Gg_KTgSMAaM6l09eDDogg-1 To: Donald Buczek , song@kernel.org References: <1613177399-22024-1-git-send-email-guoqing.jiang@cloud.ionos.com> <8312a154-14fb-6f07-0cf1-8c970187cc49@molgen.mpg.de> <87206712-b066-9d1d-3e46-14338e704c1a@linux.dev> <75958549-03d4-e396-e761-9956c4b2f991@molgen.mpg.de> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Guoqing Jiang Message-ID: <2ed2d0ba-14e9-f708-265e-8fc64902bd93@linux.dev> Date: Tue, 14 Dec 2021 18:03:43 +0800 MIME-Version: 1.0 In-Reply-To: <75958549-03d4-e396-e761-9956c4b2f991@molgen.mpg.de> X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: guoqing.jiang@linux.dev X-Mimecast-Impersonation-Protect: Policy=CLT - Impersonation Protection Definition; Similar Internal Domain=false; Similar Monitored External Domain=false; Custom External Domain=false; Mimecast External Domain=false; Newly Observed Domain=false; Internal User Name=false; Custom Display Name List=false; Reply-to Address Mismatch=false; Targeted Threat Dictionary=false; Mimecast Threat Dictionary=false; Custom Threat Dictionary=false X-Mimecast-Bulk-Signature: yes X-Mimecast-Spam-Signature: bulk X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-MIME-Autoconverted: from quoted-printable to 8bit by lists01.pubmisc.prod.ext.phx2.redhat.com id 1BEABdCj017166 X-loop: dm-devel@redhat.com Cc: linux-raid@vger.kernel.org, Paul Menzel , dm-devel@redhat.com, snitzer@redhat.com, agk@redhat.com Subject: Re: [dm-devel] [PATCH V2] md: don't unregister sync_thread with reconfig_mutex held X-BeenThere: dm-devel@redhat.com X-Mailman-Version: 2.1.12 Precedence: junk List-Id: device-mapper development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dm-devel-bounces@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Transfer-Encoding: base64 Content-Type: text/plain; charset="utf-8"; Format="flowed" CgpPbiAxMi8xNC8yMSA1OjMxIFBNLCBEb25hbGQgQnVjemVrIHdyb3RlOgoKPj4+PiDCoCAtdm9p ZCBtZF9yZWFwX3N5bmNfdGhyZWFkKHN0cnVjdCBtZGRldiAqbWRkZXYpCj4+Pj4gK3ZvaWQgbWRf cmVhcF9zeW5jX3RocmVhZChzdHJ1Y3QgbWRkZXYgKm1kZGV2LCBib29sIAo+Pj4+IHJlY29uZmln 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([91.121.223.63]:47998 "EHLO out1.migadu.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232803AbhLNKDx (ORCPT ); Tue, 14 Dec 2021 05:03:53 -0500 Subject: Re: [PATCH V2] md: don't unregister sync_thread with reconfig_mutex held DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1639476231; 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=QJqfhqj9RoOLjPpuMR7rqCOh+WalYXGdOdDYofORJSo=; b=i2zaVhRJ0otLocVstDpAX0vQTIXbK30hxuXVOjtBvIHKmjPWFOmY3R9JfIteZ8hA3gJVsP Mh2RuwfxZWLcddH4NzzGmga3BTbGBEoAitPyajLgHTtB4wEoVSBcLPo+C73c+N7TGZKb0I q4Ps+UIfchGNhqLmPDRVgLTqjc6Ux+g= To: Donald Buczek , song@kernel.org Cc: agk@redhat.com, snitzer@redhat.com, linux-raid@vger.kernel.org, dm-devel@redhat.com, Paul Menzel References: <1613177399-22024-1-git-send-email-guoqing.jiang@cloud.ionos.com> <8312a154-14fb-6f07-0cf1-8c970187cc49@molgen.mpg.de> <87206712-b066-9d1d-3e46-14338e704c1a@linux.dev> <75958549-03d4-e396-e761-9956c4b2f991@molgen.mpg.de> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Guoqing Jiang Message-ID: <2ed2d0ba-14e9-f708-265e-8fc64902bd93@linux.dev> Date: Tue, 14 Dec 2021 18:03:43 +0800 MIME-Version: 1.0 In-Reply-To: <75958549-03d4-e396-e761-9956c4b2f991@molgen.mpg.de> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: guoqing.jiang@linux.dev Precedence: bulk List-ID: X-Mailing-List: linux-raid@vger.kernel.org On 12/14/21 5:31 PM, Donald Buczek wrote: >>>>   -void md_reap_sync_thread(struct mddev *mddev) >>>> +void md_reap_sync_thread(struct mddev *mddev, bool >>>> reconfig_mutex_held) >>>>   { >>>>       struct md_rdev *rdev; >>>>       sector_t old_dev_sectors = mddev->dev_sectors; >>>>       bool is_reshaped = false; >>>>         /* resync has finished, collect result */ >>>> +    if (reconfig_mutex_held) >>>> +        mddev_unlock(mddev); >>> >>> >>> If one thread got here, e.g. via action_store( /* "idle" */ ), now >>> that the mutex is unlocked, is there anything which would prevent >>> another thread getting  here as well, e.g. via the same path? >>> >>> If not, they both might call >>> >>>> md_unregister_thread(&mddev->sync_thread); >>> >>> Which is not reentrant: >>> >>> void md_unregister_thread(struct md_thread **threadp) >>> { >>>     struct md_thread *thread = *threadp; >>>     if (!thread) >>>         return; >>>     pr_debug("interrupting MD-thread pid %d\n", >>> task_pid_nr(thread->tsk)); >>>     /* Locking ensures that mddev_unlock does not wake_up a >>>      * non-existent thread >>>      */ >>>     spin_lock(&pers_lock); >>>     *threadp = NULL; >>>     spin_unlock(&pers_lock); >>> >>>     kthread_stop(thread->tsk); >>>     kfree(thread); >>> } >>> >>> This might be a preexisting problem, because the call site in >>> dm-raid.c, which you updated to `md_reap_sync_thread(mddev, >>> false);`, didn't hold the mutex anyway. >>> >>> Am I missing something? Probably, I do. >>> >>> Otherwise: Move the deref of threadp in md_unregister_thread() into >>> the spinlock scope? >> >> Good point, I think you are right. >> >> And actually pers_lock does extra service to protect accesses to >> mddev->thread (I think it >> also suitable for mddev->sync_thread ) when the mutex can't be held. >> Care to send a patch >> for it? > > I'm really sorry, but it's one thing to point to a possible problem > and another thing to come up with a correct solution. Yes, it is often the reality of life, and we can always correct ourselves if there is problem 😎. > While I think it would be easy to avoid the double free with the > spinlock (or maybe atomic RMW) , we surely don't want to hold the > spinlock while we are sleeping in kthread_stop(). If we don't hold > some kind of lock, what are the side effects of another sync thread > being started or any other reconfiguration? Are the existing flags > enough to protect us from this? If we do want to hold the lock while > waiting for the thread to terminate, should it be made into a mutex? > If so, it probably shouldn't be static but moved into the mddev > structure. I'd need weeks if not month to figure that out and to feel > bold enough to post it. Thanks for deep thinking about it, I think we can avoid to call kthread_stop with spinlock held. Maybe something like this, but just my raw idea, please have a thorough review.  void md_unregister_thread(struct md_thread **threadp)  { -       struct md_thread *thread = *threadp; -       if (!thread) +       struct md_thread *thread = READ_ONCE(*threadp); + +       spin_lock(&pers_lock); +       if (!thread) { +               spin_unlock(&pers_lock);                 return; +       }         pr_debug("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));         /* Locking ensures that mddev_unlock does not wake_up a          * non-existent thread          */ -       spin_lock(&pers_lock);         *threadp = NULL;         spin_unlock(&pers_lock); -       kthread_stop(thread->tsk); +       if (IS_ERR_OR_NULL(thread->tsk)) { +               kthread_stop(thread->tsk); +               thread->tsk = NULL; +       }         kfree(thread);  }  EXPORT_SYMBOL(md_unregister_thread); > I don't want to push work to others, but my own my understanding of md > is to narrow. Me either 😉 Thanks, Guoqing