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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 27 Jun 2019 21:45:20 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22036.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x5RKjJY515795034 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 27 Jun 2019 20:45:19 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C2DE2B2065; Thu, 27 Jun 2019 20:45:19 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 95697B2066; Thu, 27 Jun 2019 20:45:19 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.26]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 27 Jun 2019 20:45:19 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id C05EF16C5DA5; Thu, 27 Jun 2019 13:45:21 -0700 (PDT) Date: Thu, 27 Jun 2019 13:45:21 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Sebastian Andrzej Siewior , Steven Rostedt , rcu , LKML , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Josh Triplett , Mathieu Desnoyers , Lai Jiangshan Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs Reply-To: paulmck@linux.ibm.com References: <20190626162558.GY26519@linux.ibm.com> <20190627142436.GD215968@google.com> <20190627103455.01014276@gandalf.local.home> <20190627153031.GA249127@google.com> <20190627154011.vbje64x6auaknhx4@linutronix.de> <20190627181112.GY26519@linux.ibm.com> <20190627183022.GZ26519@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190627183022.GZ26519@linux.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19062720-0064-0000-0000-000003F48ADB X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011342; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01224159; UDB=6.00644281; IPR=6.01005348; MB=3.00027495; MTD=3.00000008; XFM=3.00000015; UTC=2019-06-27 20:45:23 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19062720-0065-0000-0000-00003E0E58F5 Message-Id: <20190627204521.GA31499@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-06-27_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1906270238 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 27, 2019 at 11:30:22AM -0700, Paul E. McKenney wrote: > On Thu, Jun 27, 2019 at 11:11:12AM -0700, Paul E. McKenney wrote: > > On Thu, Jun 27, 2019 at 01:46:27PM -0400, Joel Fernandes wrote: > > > On Thu, Jun 27, 2019 at 1:43 PM Joel Fernandes wrote: > > > > > > > > On Thu, Jun 27, 2019 at 11:40 AM Sebastian Andrzej Siewior > > > > wrote: > > > > > > > > > > On 2019-06-27 11:37:10 [-0400], Joel Fernandes wrote: > > > > > > Sebastian it would be nice if possible to trace where the > > > > > > t->rcu_read_unlock_special is set for this scenario of calling > > > > > > rcu_read_unlock_special, to give a clear idea about whether it was > > > > > > really because of an IPI. I guess we could also add additional RCU > > > > > > debug fields to task_struct (just for debugging) to see where there > > > > > > unlock_special is set. > > > > > > > > > > > > Is there a test to reproduce this, or do I just boot an intel x86_64 > > > > > > machine with "threadirqs" and run into it? > > > > > > > > > > Do you want to send me a patch or should I send you my kvm image which > > > > > triggers the bug on boot? > > > > > > > > I could reproduce this as well just booting Linus tree with threadirqs > > > > command line and running rcutorture. In 15 seconds or so it locks > > > > up... gdb backtrace shows the recursive lock: > > > > > > Sorry that got badly wrapped, so I pasted it here: > > > https://hastebin.com/ajivofomik.shell > > > > Which rcutorture scenario would that be? TREE03 is thus far refusing > > to fail for me when run this way: > > > > $ tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 8 --duration 5 --trust-make --configs "TREE03" --bootargs "threadirqs" > > Ah, but I was running -rcu. TREE03 fails at 38 seconds for me on v5.2. > > Now to find out which -rcu commit fixed it. Or at least made it much > less probable, to Sebastian's point. And it still works before this one: a69987a515c8 ("rcu: Simplify rcu_read_unlock_special() deferred wakeups") (This is included in the dev branch of the -rcu tree, and is currently slated for v5.4.) And it works at these commits: 0864f057b050 ("rcu: Use irq_work to get scheduler's attention in clean context") 385b599e8c04 ("rcu: Allow rcu_read_unlock_special() to raise_softirq() if in_irq()") 25102de65fdd ("rcu: Only do rcu_read_unlock_special() wakeups if expedited") 23634ebc1d94 ("rcu: Check for wakeup-safe conditions in rcu_read_unlock_special()") I checked the last one twice, both completing without problems other than false positives due to security-induced pointer obfuscation. (These will be included in my v5.3 pull request.) But not at this commit: 48d07c04b4cc ("rcu: Enable elimination of Tree-RCU softirq processing") This gets RCU CPU stall warnings rather than the double wakeup of ksoftirqd. Works fine without traceirqs. v5.2-rc1 locks up hard at early boot (three of three attempts): [ 2.525153] clocksource: Switched to clocksource tsc [ 2.878858] random: fast init done [ 2.881122] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input3 [ 2.884183] probe of serio1 returned 1 after 671008 usecs [ 9.969474] hrtimer: interrupt took 3992554 ns v5.1 gets the double wakeup of ksoftirqd. So it looks like I need to get that pull request sent out, doesn't it? ;-) Thanx, Paul