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Subject[PATCH] sched; Simplify mutex_spin_on_owner()
It does not make sense to rcu_read_lock/unlock() in every loop
iteration while spinning on the mutex.

Move the rcu protection once outside the loop. Also simplify the
return path to always check for lock->owner == NULL which meets the
requirements of both owner changed and need_resched() caused loop
exits.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
kernel/sched.c | 25 +++++++++----------------
1 file changed, 9 insertions(+), 16 deletions(-)

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -4306,11 +4306,8 @@ EXPORT_SYMBOL(schedule);

static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
{
- bool ret = false;
-
- rcu_read_lock();
if (lock->owner != owner)
- goto fail;
+ return false;

/*
* Ensure we emit the owner->on_cpu, dereference _after_ checking
@@ -4320,11 +4317,7 @@ static inline bool owner_running(struct
*/
barrier();

- ret = owner->on_cpu;
-fail:
- rcu_read_unlock();
-
- return ret;
+ return owner->on_cpu;
}

/*
@@ -4336,21 +4329,21 @@ int mutex_spin_on_owner(struct mutex *lo
if (!sched_feat(OWNER_SPIN))
return 0;

+ rcu_read_lock();
while (owner_running(lock, owner)) {
if (need_resched())
- return 0;
+ break;

arch_mutex_cpu_relax();
}
+ rcu_read_unlock();

/*
- * If the owner changed to another task there is likely
- * heavy contention, stop spinning.
+ * We break out the loop above on need_resched() and when the
+ * owner changed, which is a sign for heavy contention. Return
+ * success only when lock->owner is NULL.
*/
- if (lock->owner)
- return 0;
-
- return 1;
+ return lock->owner == NULL;
}
#endif


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