Subject: cputimer: Cure lock inversion
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Mon Oct 17 11:50:30 CEST 2011

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>
Cc: stable@kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 kernel/posix-cpu-timers.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
Index: linux-2.6/kernel/posix-cpu-timers.c
===================================================================
--- linux-2.6.orig/kernel/posix-cpu-timers.c
+++ linux-2.6/kernel/posix-cpu-timers.c
@@ -274,9 +274,7 @@ void thread_group_cputimer(struct task_s
 	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_s
 		 * 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);
 }