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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Fri, 28 Jun 2019 19:23:26 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x5SINPpS48169446 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 28 Jun 2019 18:23:25 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 1251AB2064; Fri, 28 Jun 2019 18:23:25 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D9131B2066; Fri, 28 Jun 2019 18:23:24 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.26]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Fri, 28 Jun 2019 18:23:24 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 8F9E816C5DA7; Fri, 28 Jun 2019 11:23:25 -0700 (PDT) Date: Fri, 28 Jun 2019 11:23:25 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Steven Rostedt , Sebastian Andrzej Siewior , 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: <20190627153031.GA249127@google.com> <20190627155506.GU26519@linux.ibm.com> <20190627173831.GW26519@linux.ibm.com> <20190627181638.GA209455@google.com> <20190627184107.GA26519@linux.ibm.com> <20190628164008.GB240964@google.com> <20190628164559.GC240964@google.com> <20190628173011.GX26519@linux.ibm.com> <20190628180537.GD240964@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190628180537.GD240964@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19062818-2213-0000-0000-000003A64CA7 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011348; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01224588; UDB=6.00644540; IPR=6.01005780; MB=3.00027510; MTD=3.00000008; XFM=3.00000015; UTC=2019-06-28 18:23:28 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19062818-2214-0000-0000-00005F081ADA Message-Id: <20190628182325.GZ26519@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-06-28_08:,, 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-1906280207 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 28, 2019 at 02:05:37PM -0400, Joel Fernandes wrote: > On Fri, Jun 28, 2019 at 10:30:11AM -0700, Paul E. McKenney wrote: > > On Fri, Jun 28, 2019 at 12:45:59PM -0400, Joel Fernandes wrote: > > > On Fri, Jun 28, 2019 at 12:40:08PM -0400, Joel Fernandes wrote: > > > > On Thu, Jun 27, 2019 at 11:41:07AM -0700, Paul E. McKenney wrote: > > > > [snip] > > > > > > > > And we should document this somewhere for future sanity preservation > > > > > > > > :-D > > > > > > > > > > > > > > Or adjust the code and requirements to make it more sane, if feasible. > > > > > > > > > > > > > > My current (probably wildly unreliable) guess that the conditions in > > > > > > > rcu_read_unlock_special() need adjusting. I was assuming that in_irq() > > > > > > > implies a hardirq context, in other words that in_irq() would return > > > > > > > false from a threaded interrupt handler. If in_irq() instead returns > > > > > > > true from within a threaded interrupt handler, then this code in > > > > > > > rcu_read_unlock_special() needs fixing: > > > > > > > > > > > > > > if ((exp || in_irq()) && irqs_were_disabled && use_softirq && > > > > > > > (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) { > > > > > > > // Using softirq, safe to awaken, and we get > > > > > > > // no help from enabling irqs, unlike bh/preempt. > > > > > > > raise_softirq_irqoff(RCU_SOFTIRQ); > > > > > > > > > > > > > > The fix would be replacing the calls to in_irq() with something that > > > > > > > returns true only if called from within a hardirq context. > > > > > > > Thoughts? > > > > > > > > > > > > I am not sure if this will fix all cases though? > > > > > > > > > > > > I think the crux of the problem is doing a recursive wake up. The threaded > > > > > > IRQ probably just happens to be causing it here, it seems to me this problem > > > > > > can also occur on a non-threaded irq system (say current_reader() in your > > > > > > example executed in a scheduler path in process-context and not from an > > > > > > interrupt). Is that not possible? > > > > > > > > > > In the non-threaded case, invoking raise_softirq*() from hardirq context > > > > > just sets a bit in a per-CPU variable. Now, to Sebastian's point, we > > > > > are only sort of in hardirq context in this case due to being called > > > > > from irq_exit(), but the failure we are seeing might well be a ways > > > > > downstream of the actual root-cause bug. > > > > > > > > Hi Paul, > > > > I was talking about calling of rcu_read_unlock_special from a normal process > > > > context from the scheduler. > > > > > > > > In the below traces, it shows that only the PREEMPT_MASK offset is set at the > > > > time of the issue. Both HARD AND SOFT IRQ masks are not enabled, which means > > > > the lock up is from a normal process context. > > > > > > > > I think I finally understood why the issue shows up only with threadirqs in > > > > my setup. If I build x86_64_defconfig, the CONFIG_IRQ_FORCED_THREADING=y > > > > option is set. And booting this with threadirqs, it always tries to > > > > wakeup_ksoftirqd in invoke_softirq. > > > > > > > > I believe what happens is, at an in-opportune time when the .blocked field is > > > > set for the preempted task, an interrupt is received. This timing is quite in > > > > auspicious because t->rcu_read_unlock_special just happens to have its > > > > .blocked field set even though it is not in a reader-section. > > > > Thank you for tracing through this! > > My pleasure ;) > > > > I believe the .blocked field remains set even though we are not any more in a > > > reader section because of deferred processing of the blocked lists that you > > > mentioned yesterday. > > > > That can indeed happen. However, in current -rcu, that would mean > > that .deferred_qs is also set, which (if in_irq()) would prevent > > the raise_softirq_irqsoff() from being invoked. Which was why I was > > asking the questions about whether in_irq() returns true within threaded > > interrupts yesterday. If it does, I need to find if there is some way > > of determining whether rcu_read_unlock_special() is being called from > > a threaded interrupt in order to suppress the call to raise_softirq() > > in that case. > > Thanks. I will take a look at the -rcu tree a bit and reply to this. > > > But which version of the kernel are you using here? Current -rcu? > > v5.2-rc1? Something else? > > This is v5.2-rc6 kernel version from Linus tree which was showing the issue. OK, that version does not contain the alleged fixes that are in -rcu. Thanx, Paul