From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752331AbdCEKtH (ORCPT ); Sun, 5 Mar 2017 05:49:07 -0500 Received: from mail-wr0-f180.google.com ([209.85.128.180]:33164 "EHLO mail-wr0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751806AbdCEKtE (ORCPT ); Sun, 5 Mar 2017 05:49:04 -0500 From: Dmitry Vyukov To: paulmck@linux.vnet.ibm.com Cc: Dmitry Vyukov , syzkaller@googlegroups.com, linux-kernel@vger.kernel.org, Steven Rostedt , Mathieu Desnoyers , josh@joshtriplett.org, jiangshanlai@gmail.com Subject: [PATCH] rcu: fix warning in rcu_seq_end() Date: Sun, 5 Mar 2017 11:48:17 +0100 Message-Id: <20170305104817.47226-1-dvyukov@google.com> X-Mailer: git-send-email 2.12.0.rc1.440.g5b76565f74-goog Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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