linux-kernel - [tip:locking/core] futex: Rename barrier references in ordering guarantees

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Date:	Wed, 17 Feb 2016 04:25:39 -0800
From:	tip-bot for Davidlohr Bueso <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	clm@...com, mgorman@...hsingularity.net,
	linux-kernel@...r.kernel.org, dvhart@...ux.intel.com,
	dbueso@...e.de, dave@...olabs.net, bigeasy@...utronix.de,
	hughd@...gle.com, torvalds@...ux-foundation.org, mingo@...nel.org,
	hpa@...or.com, tglx@...utronix.de, peterz@...radead.org
Subject: [tip:locking/core] futex:
  Rename barrier references in ordering guarantees

Commit-ID:  8ad7b378d0d016309014cae0f640434bca7b5e11
Gitweb:     http://git.kernel.org/tip/8ad7b378d0d016309014cae0f640434bca7b5e11
Author:     Davidlohr Bueso <dave@...olabs.net>
AuthorDate: Tue, 9 Feb 2016 11:15:13 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 17 Feb 2016 10:42:17 +0100

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@...nel.org>
Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Chris Mason <clm@...com>
Cc: Darren Hart <dvhart@...ux.intel.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mel Gorman <mgorman@...hsingularity.net>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: dave@...olabs.net
Link: http://lkml.kernel.org/r/1455045314-8305-2-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/futex.c | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 5d6ce64..08ac700 100644
--- 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;
 }