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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 1 Aug 2019 23:31:59 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x71MVwDn47972688 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 1 Aug 2019 22:31:58 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7ECFEB206C; Thu, 1 Aug 2019 22:31:58 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4FE3AB206B; Thu, 1 Aug 2019 22:31:58 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.154]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 1 Aug 2019 22:31:58 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id A644116C9A3A; Thu, 1 Aug 2019 15:31:59 -0700 (PDT) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 2/3] rcu: Make rcu_read_unlock_special() checks match raise_softirq_irqoff() Date: Thu, 1 Aug 2019 15:31:57 -0700 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20190801223132.GA14044@linux.ibm.com> References: <20190801223132.GA14044@linux.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 19080122-0064-0000-0000-000004057838 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011535; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000287; SDB=6.01240732; UDB=6.00654293; IPR=6.01022156; MB=3.00028000; MTD=3.00000008; XFM=3.00000015; UTC=2019-08-01 22:32:03 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19080122-0065-0000-0000-00003E81E6B3 Message-Id: <20190801223158.14407-2-paulmck@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-01_09:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=15 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=8.0.1-1906280000 definitions=main-1908010237 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Threaded interrupts provide additional interesting interactions between RCU and raise_softirq() that can result in self-deadlocks in v5.0-2 of the Linux kernel. These self-deadlocks can be provoked in susceptible kernels within a few minutes using the following rcutorture command on an 8-CPU system: tools/testing/selftests/rcutorture/bin/kvm.sh --duration 5 --configs "TREE03" --bootargs "threadirqs" Although post-v5.2 RCU commits have at least greatly reduced the probability of these self-deadlocks, this was entirely by accident. Although this sort of accident should be rowdily celebrated on those rare occasions when it does occur, such celebrations should be quickly followed by a principled patch, which is what this patch purports to be. The key point behind this patch is that when in_interrupt() returns true, __raise_softirq_irqoff() will never attempt a wakeup. Therefore, if in_interrupt(), calls to raise_softirq*() are both safe and extremely cheap. This commit therefore replaces the in_irq() calls in the "if" statement in rcu_read_unlock_special() with in_interrupt() and simplifies the "if" condition to the following: if (irqs_were_disabled && use_softirq && (in_interrupt() || (exp && !t->rcu_read_unlock_special.b.deferred_qs))) { raise_softirq_irqoff(RCU_SOFTIRQ); } else { /* Appeal to the scheduler. */ } The rationale behind the "if" condition is as follows: 1. irqs_were_disabled: If interrupts are enabled, we should instead appeal to the scheduler so as to let the upcoming irq_enable()/local_bh_enable() do the rescheduling for us. 2. use_softirq: If this kernel isn't using softirq, then raise_softirq_irqoff() will be unhelpful. 3. a. in_interrupt(): If this returns true, the subsequent call to raise_softirq_irqoff() is guaranteed not to do a wakeup, so that call will be both very cheap and quite safe. b. Otherwise, if !in_interrupt() the raise_softirq_irqoff() might do a wakeup, which is expensive and, in some contexts, unsafe. i. The "exp" (an expedited RCU grace period is being blocked) says that the wakeup is worthwhile, and: ii. The !.deferred_qs says that scheduler locks cannot be held, so the wakeup will be safe. Backporting this requires considerable care, so no auto-backport, please! Fixes: 05f415715ce45 ("rcu: Speed up expedited GPs when interrupting RCU reader") Reported-by: Sebastian Andrzej Siewior Signed-off-by: Paul E. McKenney --- kernel/rcu/tree_plugin.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 3f0701e860e4..1fd3ca4ffc1d 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -626,8 +626,9 @@ static void rcu_read_unlock_special(struct task_struct *t) (rdp->grpmask & rnp->expmask) || tick_nohz_full_cpu(rdp->cpu); // Need to defer quiescent state until everything is enabled. - if ((exp || in_irq()) && irqs_were_disabled && use_softirq && - (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) { + if (irqs_were_disabled && use_softirq && + (in_interrupt() || + (exp && !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); -- 2.17.1