futex: Rename barrier references in ordering guarantees

Ingo suggested we rename how we reference barriers A and B
regarding futex ordering guarantees. This patch replaces,
for both barriers, MB (A) with smp_mb(); (A), such that:

 - We explicitly state that the barriers are SMP, and

 - We standardize how we reference these across futex.c
   helping readers follow what barrier does what and where.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Chris Mason <clm@fb.com>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: dave@stgolabs.net
Link: http://lkml.kernel.org/r/1455045314-8305-2-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/futex.c b/kernel/futex.c
index 5d6ce6413ef1d227b32c99a4330bee1289f9f571..08ac7009488bb3df4e78ef65074554268f220967 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -124,16 +124,16 @@
  *   futex_wait(futex, val);
  *
  *   waiters++; (a)
- *   mb(); (A) <-- paired with -.
- *                              |
- *   lock(hash_bucket(futex));  |
- *                              |
- *   uval = *futex;             |
- *                              |        *futex = newval;
- *                              |        sys_futex(WAKE, futex);
- *                              |          futex_wake(futex);
- *                              |
- *                              `------->  mb(); (B)
+ *   smp_mb(); (A) <-- paired with -.
+ *                                  |
+ *   lock(hash_bucket(futex));      |
+ *                                  |
+ *   uval = *futex;                 |
+ *                                  |        *futex = newval;
+ *                                  |        sys_futex(WAKE, futex);
+ *                                  |          futex_wake(futex);
+ *                                  |
+ *                                  `--------> smp_mb(); (B)
  *   if (uval == val)
  *     queue();
  *     unlock(hash_bucket(futex));
@@ -334,7 +334,7 @@ static inline void futex_get_mm(union futex_key *key)
        /*
         * Ensure futex_get_mm() implies a full barrier such that
         * get_futex_key() implies a full barrier. This is relied upon
-        * as full barrier (B), see the ordering comment above.
+        * as smp_mb(); (B), see the ordering comment above.
         */
        smp_mb__after_atomic();
 }
@@ -407,10 +407,10 @@ static void get_futex_key_refs(union futex_key *key)
 
        switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
        case FUT_OFF_INODE:
-               ihold(key->shared.inode); /* implies MB (B) */
+               ihold(key->shared.inode); /* implies smp_mb(); (B) */
                break;
        case FUT_OFF_MMSHARED:
-               futex_get_mm(key); /* implies MB (B) */
+               futex_get_mm(key); /* implies smp_mb(); (B) */
                break;
        default:
                /*
@@ -418,7 +418,7 @@ static void get_futex_key_refs(union futex_key *key)
                 * mm, therefore the only purpose of calling get_futex_key_refs
                 * is because we need the barrier for the lockless waiter check.
                 */
-               smp_mb(); /* explicit MB (B) */
+               smp_mb(); /* explicit smp_mb(); (B) */
        }
 }
 
@@ -497,7 +497,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
        if (!fshared) {
                key->private.mm = mm;
                key->private.address = address;
-               get_futex_key_refs(key);  /* implies MB (B) */
+               get_futex_key_refs(key);  /* implies smp_mb(); (B) */
                return 0;
        }
 
@@ -572,7 +572,7 @@ again:
                key->shared.pgoff = basepage_index(page);
        }
 
-       get_futex_key_refs(key); /* implies MB (B) */
+       get_futex_key_refs(key); /* implies smp_mb(); (B) */
 
 out:
        unlock_page(page);
@@ -1864,7 +1864,7 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
 
        q->lock_ptr = &hb->lock;
 
-       spin_lock(&hb->lock); /* implies MB (A) */
+       spin_lock(&hb->lock); /* implies smp_mb(); (A) */
        return hb;
 }