[tip:sched/urgent] sched_clock: Fix atomicity/continuity bug by using cmpxchg64()

[tip-bot for Eric Dumazet <eric.dumazet@gmail.com>]

Commit-ID:  152f9d0710a62708710161bce1b29fa8292c8c11
Gitweb:     http://git.kernel.org/tip/152f9d0710a62708710161bce1b29fa8292c8c11
Author:     Eric Dumazet <eric.dumazet@gmail.com>
AuthorDate: Wed, 30 Sep 2009 20:36:19 +0200
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 30 Sep 2009 22:56:10 +0200

sched_clock: Fix atomicity/continuity bug by using cmpxchg64()

Commit def0a9b2573 (sched_clock: Make it NMI safe) assumed
cmpxchg() of 64bit values was available on X86_32.

That is not so - and causes some subtle scheduler misbehavior due
to incorrect timestamps off to up by ~4 seconds.

Two symptoms are known right now:

 - interactivity problems seen by Arjan: up to 600 msecs
   latencies instead of the expected 20-40 msecs. These
   latencies are very visible on the desktop.

 - incorrect CPU stats: occasionally too high percentages in 'top',
   and crazy CPU usage stats.

Reported-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090930170754.0886ff2e@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
 kernel/sched_clock.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index ac2e1dc..479ce56 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -127,7 +127,7 @@ again:
 	clock = wrap_max(clock, min_clock);
 	clock = wrap_min(clock, max_clock);
 
-	if (cmpxchg(&scd->clock, old_clock, clock) != old_clock)
+	if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
 		goto again;
 
 	return clock;
@@ -163,7 +163,7 @@ again:
 		val = remote_clock;
 	}
 
-	if (cmpxchg(ptr, old_val, val) != old_val)
+	if (cmpxchg64(ptr, old_val, val) != old_val)
 		goto again;
 
 	return val;