cputimer: Cure lock inversion

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>

1 files changed, 4 insertions, 3 deletions

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index c8008dd58ef2..640ded8f5c48 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -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);
 }