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 --- 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. */