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Subject[087/111] mutex: Dont spin when the owner CPU is offline or other weird cases
2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>

commit 4b402210486c6414fe5fbfd85934a0a22da56b04 upstream.

Due to recent load-balancer changes that delay the task migration to
the next wakeup, the adaptive mutex spinning ends up in a live lock
when the owner's CPU gets offlined because the cpu_online() check
lives before the owner running check.

This patch changes mutex_spin_on_owner() to return 0 (don't spin) in
any case where we aren't sure about the owner struct validity or CPU
number, and if the said CPU is offline. There is no point going back &
re-evaluate spinning in corner cases like that, let's just go to
sleep.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1271212509.13059.135.camel@pasglop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/sched.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5590,7 +5590,7 @@ int mutex_spin_on_owner(struct mutex *lo
* the mutex owner just released it and exited.
*/
if (probe_kernel_address(&owner->cpu, cpu))
- goto out;
+ return 0;
#else
cpu = owner->cpu;
#endif
@@ -5600,14 +5600,14 @@ int mutex_spin_on_owner(struct mutex *lo
* the cpu field may no longer be valid.
*/
if (cpu >= nr_cpumask_bits)
- goto out;
+ return 0;

/*
* We need to validate that we can do a
* get_cpu() and that we have the percpu area.
*/
if (!cpu_online(cpu))
- goto out;
+ return 0;

rq = cpu_rq(cpu);

@@ -5626,7 +5626,7 @@ int mutex_spin_on_owner(struct mutex *lo

cpu_relax();
}
-out:
+
return 1;
}
#endif



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