1 files changed, 32 insertions, 20 deletions

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 59248dcc6ef3..69e079c5ff98 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -118,24 +118,41 @@ do {								\
 #endif
 
 /*
- * Despite its name it doesn't necessarily has to be a full barrier.
- * It should only guarantee that a STORE before the critical section
- * can not be reordered with LOADs and STOREs inside this section.
- * spin_lock() is the one-way barrier, this LOAD can not escape out
- * of the region. So the default implementation simply ensures that
- * a STORE can not move into the critical section, smp_wmb() should
- * serialize it with another STORE done by spin_lock().
+ * This barrier must provide two things:
+ *
+ *   - it must guarantee a STORE before the spin_lock() is ordered against a
+ *     LOAD after it, see the comments at its two usage sites.
+ *
+ *   - it must ensure the critical section is RCsc.
+ *
+ * The latter is important for cases where we observe values written by other
+ * CPUs in spin-loops, without barriers, while being subject to scheduling.
+ *
+ * CPU0			CPU1			CPU2
+ *
+ *			for (;;) {
+ *			  if (READ_ONCE(X))
+ *			    break;
+ *			}
+ * X=1
+ *			<sched-out>
+ *						<sched-in>
+ *						r = X;
+ *
+ * without transitivity it could be that CPU1 observes X!=0 breaks the loop,
+ * we get migrated and CPU2 sees X==0.
+ *
+ * Since most load-store architectures implement ACQUIRE with an smp_mb() after
+ * the LL/SC loop, they need no further barriers. Similarly all our TSO
+ * architectures imply an smp_mb() for each atomic instruction and equally don't
+ * need more.
+ *
+ * Architectures that can implement ACQUIRE better need to take care.
  */
-#ifndef smp_mb__before_spinlock
-#define smp_mb__before_spinlock()	smp_wmb()
+#ifndef smp_mb__after_spinlock
+#define smp_mb__after_spinlock()	do { } while (0)
 #endif
 
-/**
- * raw_spin_unlock_wait - wait until the spinlock gets unlocked
- * @lock: the spinlock in question.
- */
-#define raw_spin_unlock_wait(lock)	arch_spin_unlock_wait(&(lock)->raw_lock)
-
 #ifdef CONFIG_DEBUG_SPINLOCK
  extern void do_raw_spin_lock(raw_spinlock_t *lock) __acquires(lock);
 #define do_raw_spin_lock_flags(lock, flags) do_raw_spin_lock(lock)
@@ -369,11 +386,6 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock)
 	raw_spin_trylock_irqsave(spinlock_check(lock), flags); \
 })
 
-static __always_inline void spin_unlock_wait(spinlock_t *lock)
-{
-	raw_spin_unlock_wait(&lock->rlock);
-}
-
 static __always_inline int spin_is_locked(spinlock_t *lock)
 {
 	return raw_spin_is_locked(&lock->rlock);