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Subject[tip:sched/urgent] sched_clock: Fix atomicity/continuity bug by using cmpxchg64()
Commit-ID:  c7eca501d2ac092f2f58452a6f962249e2ab5dbe
Gitweb: http://git.kernel.org/tip/c7eca501d2ac092f2f58452a6f962249e2ab5dbe
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 22:51:51 +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;

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