mutex: Don't spin when the owner CPU is offline or other weird cases

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>

diff --git a/kernel/sched.c b/kernel/sched.c
index 6af210a..de0bd26 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3780,7 +3780,7 @@ int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
         * the mutex owner just released it and exited.
         */
        if (probe_kernel_address(&owner->cpu, cpu))
-               goto out;
+               return 0;
 #else
        cpu = owner->cpu;
 #endif
@@ -3790,14 +3790,14 @@ int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
         * 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);
 
@@ -3816,7 +3816,7 @@ int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
 
                cpu_relax();
        }
-out:
+
        return 1;
 }
 #endif