From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754262AbdBEUO1 (ORCPT + 2 others); Sun, 5 Feb 2017 15:14:27 -0500 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:27776 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752507AbdBETYD (ORCPT ); Sun, 5 Feb 2017 14:24:03 -0500 From: Willy Tarreau To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, linux@roeck-us.net Cc: Tejun Heo , Chris Worley , bfields@fieldses.org, Michael Skralivetsky , Trond Myklebust , Shaohua Li , Jeff Layton , kernel-team@fb.com, Thomas Gleixner , Mike Galbraith , Willy Tarreau Subject: [PATCH 3.10 197/319] timers: Use proper base migration in add_timer_on() Date: Sun, 5 Feb 2017 20:20:19 +0100 Message-Id: <1486322541-8206-98-git-send-email-w@1wt.eu> X-Mailer: git-send-email 2.8.0.rc2.1.gbe9624a In-Reply-To: <1486322541-8206-1-git-send-email-w@1wt.eu> References: <1486322541-8206-1-git-send-email-w@1wt.eu> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: From: Tejun Heo commit 22b886dd1018093920c4250dee2a9a3cb7cff7b8 upstream. Regardless of the previous CPU a timer was on, add_timer_on() currently simply sets timer->flags to the new CPU. As the caller must be seeing the timer as idle, this is locally fine, but the timer leaving the old base while unlocked can lead to race conditions as follows. Let's say timer was on cpu 0. cpu 0 cpu 1 ----------------------------------------------------------------------------- del_timer(timer) succeeds del_timer(timer) lock_timer_base(timer) locks cpu_0_base add_timer_on(timer, 1) spin_lock(&cpu_1_base->lock) timer->flags set to cpu_1_base operates on @timer operates on @timer This triggered with mod_delayed_work_on() which contains "if (del_timer()) add_timer_on()" sequence eventually leading to the following oops. BUG: unable to handle kernel NULL pointer dereference at (null) IP: [] detach_if_pending+0x69/0x1a0 ... Workqueue: wqthrash wqthrash_workfunc [wqthrash] task: ffff8800172ca680 ti: ffff8800172d0000 task.ti: ffff8800172d0000 RIP: 0010:[] [] detach_if_pending+0x69/0x1a0 ... Call Trace: [] del_timer+0x44/0x60 [] try_to_grab_pending+0xb6/0x160 [] mod_delayed_work_on+0x33/0x80 [] wqthrash_workfunc+0x61/0x90 [wqthrash] [] process_one_work+0x1e8/0x650 [] worker_thread+0x4e/0x450 [] kthread+0xef/0x110 [] ret_from_fork+0x3f/0x70 Fix it by updating add_timer_on() to perform proper migration as __mod_timer() does. Mike: apply tglx backport Reported-and-tested-by: Jeff Layton Signed-off-by: Tejun Heo Cc: Chris Worley Cc: bfields@fieldses.org Cc: Michael Skralivetsky Cc: Trond Myklebust Cc: Shaohua Li Cc: Jeff Layton Cc: kernel-team@fb.com Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/20151029103113.2f893924@tlielax.poochiereds.net Link: http://lkml.kernel.org/r/20151104171533.GI5749@mtj.duckdns.org Signed-off-by: Thomas Gleixner Signed-off-by: Mike Galbraith Signed-off-by: Willy Tarreau --- kernel/timer.c | 19 ++++++++++++++++--- 1 file changed, 16 insertions(+), 3 deletions(-) diff --git a/kernel/timer.c b/kernel/timer.c index 20f45ea..be22e45 100644 --- a/kernel/timer.c +++ b/kernel/timer.c @@ -923,13 +923,26 @@ EXPORT_SYMBOL(add_timer); */ void add_timer_on(struct timer_list *timer, int cpu) { - struct tvec_base *base = per_cpu(tvec_bases, cpu); + struct tvec_base *new_base = per_cpu(tvec_bases, cpu); + struct tvec_base *base; unsigned long flags; timer_stats_timer_set_start_info(timer); BUG_ON(timer_pending(timer) || !timer->function); - spin_lock_irqsave(&base->lock, flags); - timer_set_base(timer, base); + + /* + * If @timer was on a different CPU, it should be migrated with the + * old base locked to prevent other operations proceeding with the + * wrong base locked. See lock_timer_base(). + */ + base = lock_timer_base(timer, &flags); + if (base != new_base) { + timer_set_base(timer, NULL); + spin_unlock(&base->lock); + base = new_base; + spin_lock(&base->lock); + timer_set_base(timer, base); + } debug_activate(timer, timer->expires); internal_add_timer(base, timer); /* -- 2.8.0.rc2.1.gbe9624a