Patch "sched: Fix hrtimer_cancel()/rq->lock deadlock" has been added to the 3.4-stable tree

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This is a note to let you know that I've just added the patch titled

    sched: Fix hrtimer_cancel()/rq->lock deadlock

to the 3.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     sched-fix-hrtimer_cancel-rq-lock-deadlock.patch
and it can be found in the queue-3.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 927b54fccbf04207ec92f669dce6806848cbec7d Mon Sep 17 00:00:00 2001
From: Ben Segall <bsegall@xxxxxxxxxx>
Date: Wed, 16 Oct 2013 11:16:22 -0700
Subject: sched: Fix hrtimer_cancel()/rq->lock deadlock

From: Ben Segall <bsegall@xxxxxxxxxx>

commit 927b54fccbf04207ec92f669dce6806848cbec7d upstream.

__start_cfs_bandwidth calls hrtimer_cancel while holding rq->lock,
waiting for the hrtimer to finish. However, if sched_cfs_period_timer
runs for another loop iteration, the hrtimer can attempt to take
rq->lock, resulting in deadlock.

Fix this by ensuring that cfs_b->timer_active is cleared only if the
_latest_ call to do_sched_cfs_period_timer is returning as idle. Then
__start_cfs_bandwidth can just call hrtimer_try_to_cancel and wait for
that to succeed or timer_active == 1.

Signed-off-by: Ben Segall <bsegall@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: pjt@xxxxxxxxxx
Link: http://lkml.kernel.org/r/20131016181622.22647.16643.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Chris J Arges <chris.j.arges@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/sched/fair.c |   15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1771,6 +1771,13 @@ static int do_sched_cfs_period_timer(str
 	if (idle)
 		goto out_unlock;
 
+	/*
+	 * if we have relooped after returning idle once, we need to update our
+	 * status as actually running, so that other cpus doing
+	 * __start_cfs_bandwidth will stop trying to cancel us.
+	 */
+	cfs_b->timer_active = 1;
+
 	__refill_cfs_bandwidth_runtime(cfs_b);
 
 	if (!throttled) {
@@ -2043,11 +2050,11 @@ void __start_cfs_bandwidth(struct cfs_ba
 	 * (timer_active==0 becomes visible before the hrtimer call-back
 	 * terminates).  In either case we ensure that it's re-programmed
 	 */
-	while (unlikely(hrtimer_active(&cfs_b->period_timer))) {
+	while (unlikely(hrtimer_active(&cfs_b->period_timer)) &&
+	       hrtimer_try_to_cancel(&cfs_b->period_timer) < 0) {
+		/* bounce the lock to allow do_sched_cfs_period_timer to run */
 		raw_spin_unlock(&cfs_b->lock);
-		/* ensure cfs_b->lock is available while we wait */
-		hrtimer_cancel(&cfs_b->period_timer);
-
+		cpu_relax();
 		raw_spin_lock(&cfs_b->lock);
 		/* if someone else restarted the timer then we're done */
 		if (cfs_b->timer_active)


Patches currently in stable-queue which might be from bsegall@xxxxxxxxxx are

queue-3.4/sched-fix-hrtimer_cancel-rq-lock-deadlock.patch
queue-3.4/sched-fix-cfs_bandwidth-misuse-of-hrtimer_expires_remaining.patch
queue-3.4/sched-fix-race-on-toggling-cfs_bandwidth_used.patch
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