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=-16.4 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 BE528C4363A for ; Mon, 26 Oct 2020 20:53:32 +0000 (UTC) Received: from lists.lttng.org (lists.lttng.org [167.114.26.123]) (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 E054A20829 for ; Mon, 26 Oct 2020 20:53:31 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.lttng.org header.i=@lists.lttng.org header.b="ioGF4kTr" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E054A20829 Authentication-Results: mail.kernel.org; dmarc=pass (p=none dis=none) header.from=lists.lttng.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=lttng-dev-bounces@lists.lttng.org Received: from lists-lttng01.efficios.com (localhost [IPv6:::1]) by lists.lttng.org (Postfix) with ESMTP id 4CKn9k1cn8z17Q1; Mon, 26 Oct 2020 16:53:30 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=lists.lttng.org; s=default; t=1603745610; bh=ioh8CScYhw3ZH5M3iZETYM0ELgc3W4M2iZ1xIRq+0Ms=; h=Date:To:Cc:References:In-Reply-To:Subject:List-Id: List-Unsubscribe:List-Archive:List-Post:List-Help:List-Subscribe: From:Reply-To:From; b=ioGF4kTrx/dwp6TFokZ0DbtpSMQf/G16c6Va70xzNQmCMeyc68SzmWml/0sgtcRsP Pn98RRBuFBwIBlkOVU1UlUA8jjqeN0HBYxWX/3wptaQ+iEtX1bFl32fSL4Jwf2Amt6 luevUz2E9ciq5YNcYJDX3epoQRNYyr7GrYCleEq/2iwCcfAFJnZJ1bc/ymPp5PeyiO Msp4BlhNV2ePQh3RkK9aQRy2SB1pMq/NQIqzKJoP+tm5lQ0/jvWX3HM0APwlXkKx7Y YdiKPnPu6Xgb/tqptl0WuPUFkFzGrBrCsQpqdBj1uB+o8bsah4CarYYcy/DPB2eSV0 HMmMg5W/VGdZw== Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by lists.lttng.org (Postfix) with ESMTPS id 4CKn9h1SRhz17Gv for ; Mon, 26 Oct 2020 16:53:28 -0400 (EDT) Received: from paulmck-ThinkPad-P72.home (50-39-104-11.bvtn.or.frontiernet.net [50.39.104.11]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id C29E720829; Mon, 26 Oct 2020 20:53:20 +0000 (UTC) Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 6ABA335207A6; Mon, 26 Oct 2020 13:53:20 -0700 (PDT) Date: Mon, 26 Oct 2020 13:53:20 -0700 To: Mathieu Desnoyers Cc: Stephen Hemminger , Alan Stern , Lai Jiangshan , lttng-dev , linux-kernel Message-ID: <20201026205320.GT3249@paulmck-ThinkPad-P72> References: <20201022223021.GA8535@paulmck-ThinkPad-P72> <1576751762.38206.1603742291604.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <1576751762.38206.1603742291604.JavaMail.zimbra@efficios.com> User-Agent: Mutt/1.9.4 (2018-02-28) Subject: Re: [lttng-dev] [PATCH] call_rcu: Fix race between rcu_barrier() and call_rcu_data_free() X-BeenThere: lttng-dev@lists.lttng.org X-Mailman-Version: 2.1.31 Precedence: list List-Id: LTTng development list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: "Paul E. McKenney via lttng-dev" Reply-To: paulmck@kernel.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: lttng-dev-bounces@lists.lttng.org Sender: "lttng-dev" On Mon, Oct 26, 2020 at 03:58:11PM -0400, Mathieu Desnoyers wrote: > ----- On Oct 22, 2020, at 6:30 PM, paulmck paulmck@kernel.org wrote: > > > The current code can lose RCU callbacks at shutdown time, which can > > result in hangs. This lossage can happen as follows: > > > > o A thread invokes call_rcu_data_free(), which executes up through > > the wake_call_rcu_thread(). At this point, the call_rcu_data > > structure has been drained of callbacks, but is still on the > > call_rcu_data_list. Note that this thread does not hold the > > call_rcu_mutex. > > > > o Another thread invokes rcu_barrier(), which traverses the > > call_rcu_data_list under the protection of call_rcu_mutex, > > a list which still includes the above newly drained structure. > > This thread therefore adds a callback to the newly drained > > call_rcu_data structure. It then releases call_rcu_mutex and > > enters a mystifying loop that does futex stuff. > > > > o The first thread finishes executing call_rcu_data_free(), > > which acquires call_rcu_mutex just long enough to remove the > > newly drained call_rcu_data structure from call_rcu_data_list. > > Which causes one of the rcu_barrier() invocation's callbacks to > > be leaked. > > > > o The second thread's rcu_barrier() invocation never returns > > resulting in a hang. > > > > This commit therefore changes call_rcu_data_free() to acquire > > call_rcu_mutex before checking the call_rcu_data structure for callbacks. > > In the case where there are no callbacks, call_rcu_mutex is held across > > both the check and the removal from call_rcu_data_list, thus preventing > > rcu_barrier() from adding a callback in the meantime. In the case where > > there are callbacks, call_rcu_mutex must be momentarily dropped across > > the call to get_default_call_rcu_data(), which can itself acquire > > call_rcu_mutex. This momentary drop is not a problem because any > > callbacks that rcu_barrier() might queue during that period of time will > > be moved to the default call_rcu_data structure, and the lock will be > > held across the full time including moving those callbacks and removing > > the call_rcu_data structure that was passed into call_rcu_data_free() > > from call_rcu_data_list. > > > > With this fix, a several-hundred-CPU test successfully completes more > > than 5,000 executions. Without this fix, it fails within a few tens > > of executions. Although the failures happen more quickly on larger > > systems, in theory this could happen on a single-CPU system, courtesy > > of preemption. > > I agree with this fix, will merge in liburcu master, stable-0.12, and stable-2.11. > Out of curiosity, which test is hanging ? Is it a test which is part of the liburcu > tree or some out-of-tree test ? I wonder why we did not catch it in our CI [1]. The hung test was from perfbook [1] in the CodeSamples/datastruct/hash directory. A repeat-by is as follows: # Have userspace RCU preinstalled as you wish. git clone git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git cd CodeSamples make pthreads cd datastruct/hash make time for ((i = 0; i < 2000; i++)); do echo $i; ./hash_bkt_rcu --schroedinger --nreaders 444 --nupdaters 4 --duration 1000 --updatewait 1 --nbuckets 262144 --elems/writer 65536; done This normally hangs within a few tens of iterations. With this patch, the passes more than 6,000 iterations. I have smaller tests that produce this same hang on my 12-CPU laptop, but with much lower probability. Here is one example that did hang on my laptop, and which could be placed into a similar bash loop as above: hash_bkt_rcu --schroedinger --nreaders 10 --nupdaters 2 --duration 1000 --updatewait 1 --nbuckets 8192 --elems/writer 4096 But I don't have a good estimate of the hang probability, except a suspicion that it is lower than would be convenient for a CI test. Attaching to the hung process using gdb did confirm the type of hang, however. It might be possible to create a focused test that races rcu_barrier() against thread exit, where threads are created and exit repeatedly, and make a per-thread call_rcu() worker in the meantime.. Thoughts? Thanx, Paul [1] git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git > Thanks, > > Mathieu > > [1] https://ci.lttng.org/view/Liburcu/ > > > > > Signed-off-by: Paul E. McKenney > > Cc: Mathieu Desnoyers > > Cc: Stephen Hemminger > > Cc: Alan Stern > > Cc: Lai Jiangshan > > Cc: > > Cc: > > > > --- > > > > urcu-call-rcu-impl.h | 7 +++++-- > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > diff --git a/src/urcu-call-rcu-impl.h b/src/urcu-call-rcu-impl.h > > index b6ec6ba..18fd65a 100644 > > --- a/src/urcu-call-rcu-impl.h > > +++ b/src/urcu-call-rcu-impl.h > > @@ -772,9 +772,13 @@ void call_rcu_data_free(struct call_rcu_data *crdp) > > while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0) > > (void) poll(NULL, 0, 1); > > } > > + call_rcu_lock(&call_rcu_mutex); > > if (!cds_wfcq_empty(&crdp->cbs_head, &crdp->cbs_tail)) { > > - /* Create default call rcu data if need be */ > > + call_rcu_unlock(&call_rcu_mutex); > > + /* Create default call rcu data if need be. */ > > + /* CBs queued here will be handed to the default list. */ > > (void) get_default_call_rcu_data(); > > + call_rcu_lock(&call_rcu_mutex); > > __cds_wfcq_splice_blocking(&default_call_rcu_data->cbs_head, > > &default_call_rcu_data->cbs_tail, > > &crdp->cbs_head, &crdp->cbs_tail); > > @@ -783,7 +787,6 @@ void call_rcu_data_free(struct call_rcu_data *crdp) > > wake_call_rcu_thread(default_call_rcu_data); > > } > > > > - call_rcu_lock(&call_rcu_mutex); > > cds_list_del(&crdp->list); > > call_rcu_unlock(&call_rcu_mutex); > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev