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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Fri, 28 Jun 2019 19:22:16 +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 x5SIMFeX13041942 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 28 Jun 2019 18:22:15 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B330AB205F; Fri, 28 Jun 2019 18:22:15 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8622AB2064; Fri, 28 Jun 2019 18:22:15 +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 18:22:15 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 3AAA616C5DE8; Fri, 28 Jun 2019 11:22:16 -0700 (PDT) Date: Fri, 28 Jun 2019 11:22:16 -0700 From: "Paul E. McKenney" To: Sebastian Andrzej Siewior Cc: Joel Fernandes , 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: <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> <20190628174545.pwgwi3wxl2eapkvm@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190628174545.pwgwi3wxl2eapkvm@linutronix.de> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19062818-0060-0000-0000-00000356C6B3 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011347; 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 18:22:19 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19062818-0061-0000-0000-000049F12615 Message-Id: <20190628182216.GY26519@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 07:45:45PM +0200, Sebastian Andrzej Siewior wrote: > On 2019-06-28 10:30:11 [-0700], Paul E. McKenney wrote: > > > 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. > > Please not that: > | void irq_exit(void) > | { > |… > in_irq() returns true > | preempt_count_sub(HARDIRQ_OFFSET); > in_irq() returns false > | if (!in_interrupt() && local_softirq_pending()) > | invoke_softirq(); > > -> invoke_softirq() does > | if (!force_irqthreads) { > | __do_softirq(); > | } else { > | wakeup_softirqd(); > | } > > so for `force_irqthreads' rcu_read_unlock_special() within > wakeup_softirqd() will see false. OK, fair point. How about the following instead, again on -rcu? Here is the rationale for the new version of the "if" statement: 1. irqs_were_disabled: If interrupts are enabled, we should instead let the upcoming irq_enable()/local_bh_enable() do the rescheduling for us. 2. use_softirq: If we aren't using softirq, then raise_softirq_irqoff() will be unhelpful. 3a. 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. 3b. Otherwise, if !in_interrupt(), if exp (an expedited RCU grace period is being blocked), then incurring wakeup overhead is worthwhile, and if also !.deferred_qs then scheduler locks cannot be held so the wakeup will be safe. Does that make more sense? Thanx, Paul ------------------------------------------------------------------------ diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 82c925df1d92..83333cfe8707 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -624,8 +624,9 @@ 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);