linux-kernel - [PATCH RT 3/4] timers: Move clearing of base::timer

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Date:   Fri, 11 Dec 2020 16:41:06 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Tom Zanussi <zanussi@...nel.org>,
        "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>,
        syzbot+aa7c2385d46c5eba0b89@...kaller.appspotmail.com,
        syzbot+abea4558531bae1ba9fe@...kaller.appspotmail.com,
        stable-rt@...r.kernel.org
Subject: [PATCH RT 3/4] timers: Move clearing of base::timer_running under base::lock

5.4.82-rt46-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@...utronix.de>

syzbot reported KCSAN data races vs. timer_base::timer_running being set to
NULL without holding base::lock in expire_timers().

This looks innocent and most reads are clearly not problematic but for a
non-RT kernel it's completely irrelevant whether the store happens before
or after taking the lock. For an RT kernel moving the store under the lock
requires an extra unlock/lock pair in the case that there is a waiter for
the timer. But that's not the end of the world and definitely not worth the
trouble of adding boatloads of comments and annotations to the code. Famous
last words...

Reported-by: syzbot+aa7c2385d46c5eba0b89@...kaller.appspotmail.com
Reported-by: syzbot+abea4558531bae1ba9fe@...kaller.appspotmail.com
Link: https://lkml.kernel.org/r/87lfea7gw8.fsf@nanos.tec.linutronix.de
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: stable-rt@...r.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 kernel/time/timer.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 15b838401af8..86bb218d1df5 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1269,8 +1269,10 @@ static inline void timer_base_unlock_expiry(struct timer_base *base)
 static void timer_sync_wait_running(struct timer_base *base)
 {
 	if (atomic_read(&base->timer_waiters)) {
+		raw_spin_unlock_irq(&base->lock);
 		spin_unlock(&base->expiry_lock);
 		spin_lock(&base->expiry_lock);
+		raw_spin_lock_irq(&base->lock);
 	}
 }
 
@@ -1461,14 +1463,14 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
 		if (timer->flags & TIMER_IRQSAFE) {
 			raw_spin_unlock(&base->lock);
 			call_timer_fn(timer, fn, baseclk);
-			base->running_timer = NULL;
 			raw_spin_lock(&base->lock);
+			base->running_timer = NULL;
 		} else {
 			raw_spin_unlock_irq(&base->lock);
 			call_timer_fn(timer, fn, baseclk);
+			raw_spin_lock_irq(&base->lock);
 			base->running_timer = NULL;
 			timer_sync_wait_running(base);
-			raw_spin_lock_irq(&base->lock);
 		}
 	}
 }
-- 
2.29.2