cputimer: Cure lock inversion
Peter Zijlstra [Mon, 17 Oct 2011 09:50:30 +0000 (11:50 +0200)]
There's a lock inversion between the cputimer->lock and rq->lock;
notably the two callchains involved are:

 update_rlimit_cpu()
   sighand->siglock
   set_process_cpu_timer()
     cpu_timer_sample_group()
       thread_group_cputimer()
         cputimer->lock
         thread_group_cputime()
           task_sched_runtime()
             ->pi_lock
             rq->lock

 scheduler_tick()
   rq->lock
   task_tick_fair()
     update_curr()
       account_group_exec()
         cputimer->lock

Where the first one is enabling a CLOCK_PROCESS_CPUTIME_ID timer, and
the second one is keeping up-to-date.

This problem was introduced by e8abccb7193 ("posix-cpu-timers: Cure
SMP accounting oddities").

Cure the problem by removing the cputimer->lock and rq->lock nesting,
this leaves concurrent enablers doing duplicate work, but the time
wasted should be on the same order otherwise wasted spinning on the
lock and the greater-than assignment filter should ensure we preserve
monotonicity.

Reported-by: Dave Jones <davej@redhat.com>
Reported-by: Simon Kirby <sim@hostway.ca>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: stable@kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Link: http://lkml.kernel.org/r/1318928713.21167.4.camel@twins
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/posix-cpu-timers.c

index c8008dd..640ded8 100644 (file)
@@ -274,9 +274,7 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
        struct task_cputime sum;
        unsigned long flags;
 
-       spin_lock_irqsave(&cputimer->lock, flags);
        if (!cputimer->running) {
-               cputimer->running = 1;
                /*
                 * The POSIX timer interface allows for absolute time expiry
                 * values through the TIMER_ABSTIME flag, therefore we have
@@ -284,8 +282,11 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
                 * it.
                 */
                thread_group_cputime(tsk, &sum);
+               spin_lock_irqsave(&cputimer->lock, flags);
+               cputimer->running = 1;
                update_gt_cputime(&cputimer->cputime, &sum);
-       }
+       } else
+               spin_lock_irqsave(&cputimer->lock, flags);
        *times = cputimer->cputime;
        spin_unlock_irqrestore(&cputimer->lock, flags);
 }