From: Frederic Weisbecker <frederic@xxxxxxxxxx>
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@xxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Fixes: 60f2ceaa8111 ("posix-cpu-timers: Remove unnecessary locking around cpu_clock_sample_group")
Cc: stable@xxxxxxxxxxxxxxx
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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.
*/
