| From 1a3402d93c73bf6bb4df6d7c2aac35abfc3c50e2 Mon Sep 17 00:00:00 2001 |
| From: Frederic Weisbecker <frederic@kernel.org> |
| Date: Thu, 3 Jun 2021 01:15:59 +0200 |
| Subject: posix-cpu-timers: Fix rearm racing against process tick |
| |
| From: Frederic Weisbecker <frederic@kernel.org> |
| |
| commit 1a3402d93c73bf6bb4df6d7c2aac35abfc3c50e2 upstream. |
| |
| 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. |
| |
| Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> |
| Signed-off-by: Frederic Weisbecker <frederic@kernel.org> |
| Fixes: 60f2ceaa8111 ("posix-cpu-timers: Remove unnecessary locking around cpu_clock_sample_group") |
| Cc: stable@vger.kernel.org |
| Cc: Oleg Nesterov <oleg@redhat.com> |
| Cc: Thomas Gleixner <tglx@linutronix.de> |
| Cc: Ingo Molnar <mingo@kernel.org> |
| Cc: Eric W. Biederman <ebiederm@xmission.com> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| --- |
| kernel/time/posix-cpu-timers.c | 10 +++++----- |
| 1 file changed, 5 insertions(+), 5 deletions(-) |
| |
| --- 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 |
| 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 |
| |
| 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. |
| */ |