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 cd2593e1a3ec..55cc50323ce1 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