mutex: Use p->on_cpu for the adaptive spin

Since we now have p->on_cpu unconditionally available, use it to
re-implement mutex_spin_on_owner.

Requested-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frank Rowand <frank.rowand@am.sony.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20110405152728.826338173@chello.nl

1 files changed, 33 insertions, 50 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index cd2593e1a3e..55cc50323ce 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4173,70 +4173,53 @@ need_resched:
 EXPORT_SYMBOL(schedule);
 
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
-/*
- * Look out! "owner" is an entirely speculative pointer
- * access and not reliable.
- */
-int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
-{
-	unsigned int cpu;
-	struct rq *rq;
 
-	if (!sched_feat(OWNER_SPIN))
-		return 0;
+static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
+{
+	bool ret = false;
 
-#ifdef CONFIG_DEBUG_PAGEALLOC
-	/*
-	 * Need to access the cpu field knowing that
-	 * DEBUG_PAGEALLOC could have unmapped it if
-	 * the mutex owner just released it and exited.
-	 */
-	if (probe_kernel_address(&owner->cpu, cpu))
-		return 0;
-#else
-	cpu = owner->cpu;
-#endif
+	rcu_read_lock();
+	if (lock->owner != owner)
+		goto fail;
 
 	/*
-	 * Even if the access succeeded (likely case),
-	 * the cpu field may no longer be valid.
+	 * Ensure we emit the owner->on_cpu, dereference _after_ checking
+	 * lock->owner still matches owner, if that fails, owner might
+	 * point to free()d memory, if it still matches, the rcu_read_lock()
+	 * ensures the memory stays valid.
 	 */
-	if (cpu >= nr_cpumask_bits)
-		return 0;
+	barrier();
 
-	/*
-	 * We need to validate that we can do a
-	 * get_cpu() and that we have the percpu area.
-	 */
-	if (!cpu_online(cpu))
-		return 0;
+	ret = owner->on_cpu;
+fail:
+	rcu_read_unlock();
 
-	rq = cpu_rq(cpu);
+	return ret;
+}
 
-	for (;;) {
-		/*
-		 * Owner changed, break to re-assess state.
-		 */
-		if (lock->owner != owner) {
-			/*
-			 * If the lock has switched to a different owner,
-			 * we likely have heavy contention. Return 0 to quit
-			 * optimistic spinning and not contend further:
-			 */
-			if (lock->owner)
-				return 0;
-			break;
-		}
+/*
+ * Look out! "owner" is an entirely speculative pointer
+ * access and not reliable.
+ */
+int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
+{
+	if (!sched_feat(OWNER_SPIN))
+		return 0;
 
-		/*
-		 * Is that owner really running on that cpu?
-		 */
-		if (task_thread_info(rq->curr) != owner || need_resched())
+	while (owner_running(lock, owner)) {
+		if (need_resched())
 			return 0;
 
 		arch_mutex_cpu_relax();
 	}
 
+	/*
+	 * If the owner changed to another task there is likely
+	 * heavy contention, stop spinning.
+	 */
+	if (lock->owner)
+		return 0;
+
 	return 1;
 }
 #endif