[PATCH 2/3] locking: Clarify requirements for smp_mb__after_spinlock()

[Andrea Parri]

There are 11 interpretations of the requirements described in the header
comment for smp_mb__after_spinlock(): one for each LKMM maintainer, and
one currently encoded in the Cat file. Stick to the latter (until a more
satisfactory solution is presented/agreed).

Signed-off-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
---
 include/linux/spinlock.h | 25 ++-----------------------
 1 file changed, 2 insertions(+), 23 deletions(-)

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 1e8a464358384..6737ee2381d50 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -114,29 +114,8 @@ do {								\
 #endif /*arch_spin_is_contended*/
 
 /*
- * 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.
+ * smp_mb__after_spinlock() provides a full memory barrier between po-earlier
+ * lock acquisitions and po-later memory accesses.
  *
  * 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
-- 
2.7.4