posix-cpu-timers: Fix rearm racing against process tick

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) <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Fixes: 60f2ceaa8111 ("posix-cpu-timers: Remove unnecessary locking around cpu_clock_sample_group")
Cc: [email protected]
Cc: Oleg Nesterov <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Eric W. Biederman <[email protected]>

0 files changed, 0 insertions, 0 deletions