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From: Frederic Weisbecker <frederic@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>
Subject: [PATCH 1/6] posix-cpu-timers: Fix rearm racing against process tick
Date: Fri,  4 Jun 2021 13:31:54 +0200	[thread overview]
Message-ID: <20210604113159.26177-2-frederic@kernel.org> (raw)
In-Reply-To: <20210604113159.26177-1-frederic@kernel.org>

Since the process wide cputime counter is started locklessly from
posix_cpu_timer_rearm(), it can be concurrently stopped by operations
on other timers from the same thread group, such as in the following
unlucky scenario:

         CPU 0                                CPU 1
         -----                                -----
                                           timer_settime(TIMER B)
   posix_cpu_timer_rearm(TIMER A)
       cpu_clock_sample_group()
           (pct->timers_active already true)

                                           handle_posix_cpu_timers()
                                               check_process_timers()
                                                   stop_process_timers()
                                                       pct->timers_active = false
       arm_timer(TIMER A)

   tick -> run_posix_cpu_timers()
       // sees !pct->timers_active, ignore
       // our TIMER A

Fix this with simply locking process wide cputime counting start and
timer arm in the same block.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
---
 kernel/time/posix-cpu-timers.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 3bb96a8b49c9..aa52fc85dbcb 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -991,6 +991,11 @@ static void posix_cpu_timer_rearm(struct k_itimer *timer)
 	if (!p)
 		goto out;
 
+	/* Protect timer list r/w in arm_timer() */
+	sighand = lock_task_sighand(p, &flags);
+	if (unlikely(sighand == NULL))
+		goto out;
+
 	/*
 	 * Fetch the current sample and update the timer's expiry time.
 	 */
@@ -1001,11 +1006,6 @@ static void posix_cpu_timer_rearm(struct k_itimer *timer)
 
 	bump_cpu_timer(timer, now);
 
-	/* Protect timer list r/w in arm_timer() */
-	sighand = lock_task_sighand(p, &flags);
-	if (unlikely(sighand == NULL))
-		goto out;
-
 	/*
 	 * Now re-arm for the new expiry time.
 	 */
-- 
2.25.1


  reply	other threads:[~2021-06-04 11:32 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-04 11:31 [PATCH 0/6] posix-cpu-timers: Bunch of fixes Frederic Weisbecker
2021-06-04 11:31 ` Frederic Weisbecker [this message]
2021-06-09 11:54   ` [PATCH 1/6] posix-cpu-timers: Fix rearm racing against process tick Frederic Weisbecker
2021-06-11 11:49     ` Peter Zijlstra
2021-06-11 12:37       ` Frederic Weisbecker
2021-06-04 11:31 ` [PATCH 2/6] posix-cpu-timers: Don't start process wide cputime counter if timer is disabled Frederic Weisbecker
2021-06-09 12:18   ` Frederic Weisbecker
2021-06-10 10:24     ` Frederic Weisbecker
2021-06-16  8:51   ` Peter Zijlstra
2021-06-16 10:51     ` Frederic Weisbecker
2021-06-16 11:26       ` Peter Zijlstra
2021-06-16 11:50         ` Peter Zijlstra
2021-06-04 11:31 ` [PATCH 3/6] posix-cpu-timers: Force next_expiration recalc after timer deletion Frederic Weisbecker
2021-06-16  9:16   ` Peter Zijlstra
2021-06-04 11:31 ` [PATCH 4/6] posix-cpu-timers: Force next_expiration recalc after timer reset Frederic Weisbecker
2021-06-16  9:23   ` Peter Zijlstra
2021-06-16 11:21     ` Frederic Weisbecker
2021-06-16 11:33       ` Peter Zijlstra
2021-06-04 11:31 ` [PATCH 5/6] posix-cpu-timers: Force next expiration recalc after early timer firing Frederic Weisbecker
2021-06-16  9:42   ` Peter Zijlstra
2021-06-16 11:59     ` Frederic Weisbecker
2021-06-16 13:23       ` Peter Zijlstra
2021-06-16 14:53         ` Frederic Weisbecker
2021-06-04 11:31 ` [PATCH 6/6] posix-cpu-timers: Force next expiration recalc after itimer reset Frederic Weisbecker

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