From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752537AbdCEUU1 (ORCPT ); Sun, 5 Mar 2017 15:20:27 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:34300 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752174AbdCEUU0 (ORCPT ); Sun, 5 Mar 2017 15:20:26 -0500 Date: Sun, 5 Mar 2017 12:20:18 -0800 From: "Paul E. McKenney" To: Dmitry Vyukov Cc: syzkaller@googlegroups.com, linux-kernel@vger.kernel.org, Steven Rostedt , Mathieu Desnoyers , josh@joshtriplett.org, jiangshanlai@gmail.com Subject: Re: [PATCH] rcu: fix warning in rcu_seq_end() Reply-To: paulmck@linux.vnet.ibm.com References: <20170305104817.47226-1-dvyukov@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170305104817.47226-1-dvyukov@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Content-Scanned: Fidelis XPS MAILER x-cbid: 17030520-0008-0000-0000-0000076A2CD8 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006728; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000206; SDB=6.00830419; UDB=6.00407321; IPR=6.00608010; BA=6.00005188; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00014521; XFM=3.00000012; UTC=2017-03-05 20:20:22 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17030520-0009-0000-0000-00004076BA56 Message-Id: <20170305202018.GE30506@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-03-05_14:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1702020001 definitions=main-1703050176 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Mar 05, 2017 at 11:48:17AM +0100, Dmitry Vyukov wrote: > rcu_seq_end() increments seq signifying completion of a grace period, > after that checks that the seq is even and wakes > _synchronize_rcu_expedited(). _synchronize_rcu_expedited() uses > wait_event() to wait for even seq. The problem is that wait_event() > can return as soon as seq becomes even without waiting for the wakeup. > In such case the warning in rcu_seq_end() can falsely fire if the next > expedited grace period starts before the check. > > Check that seq has good value before incrementing it. > > Signed-off-by: Dmitry Vyukov > Cc: syzkaller@googlegroups.com > Cc: linux-kernel@vger.kernel.org > Cc: Paul E. McKenney > Cc: Steven Rostedt > Cc: Mathieu Desnoyers > Cc: josh@joshtriplett.org > Cc: jiangshanlai@gmail.com Queued for testing and review, thank you! Thanx, Paul > --- > > syzkaller-triggered warning: > > WARNING: CPU: 0 PID: 4832 at kernel/rcu/tree.c:3533 > rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533 > CPU: 0 PID: 4832 Comm: kworker/0:3 Not tainted 4.10.0+ #276 > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 > Workqueue: events wait_rcu_exp_gp > Call Trace: > __dump_stack lib/dump_stack.c:15 [inline] > dump_stack+0x2ee/0x3ef lib/dump_stack.c:51 > panic+0x1fb/0x412 kernel/panic.c:179 > __warn+0x1c4/0x1e0 kernel/panic.c:540 > warn_slowpath_null+0x2c/0x40 kernel/panic.c:583 > rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533 > rcu_exp_gp_seq_end kernel/rcu/tree_exp.h:36 [inline] > rcu_exp_wait_wake+0x8a9/0x1330 kernel/rcu/tree_exp.h:517 > rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:559 [inline] > wait_rcu_exp_gp+0x83/0xc0 kernel/rcu/tree_exp.h:570 > process_one_work+0xc06/0x1c20 kernel/workqueue.c:2096 > worker_thread+0x223/0x19c0 kernel/workqueue.c:2230 > kthread+0x326/0x3f0 kernel/kthread.c:227 > ret_from_fork+0x31/0x40 arch/x86/entry/entry_64.S:430 > --- > kernel/rcu/tree.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 50fee7689e71..47339b242423 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3531,8 +3531,8 @@ static void rcu_seq_start(unsigned long *sp) > static void rcu_seq_end(unsigned long *sp) > { > smp_mb(); /* Ensure update-side operation before counter increment. */ > + WARN_ON_ONCE(!(*sp & 0x1)); > WRITE_ONCE(*sp, *sp + 1); > - WARN_ON_ONCE(*sp & 0x1); > } > > /* Take a snapshot of the update side's sequence number. */ > -- > 2.12.0.rc1.440.g5b76565f74-goog >