[PATCH] futex: Mention key referencing differences between shared and private futexes

[Davidlohr Bueso <dave@stgolabs.net>]

From: Davidlohr Bueso <dave@stgolabs.net>

Update our documentation as of fix 76835b0ebf8 (futex: Ensure
get_futex_key_refs() always implies a barrier). Explicitly
state that we don't do key referencing for private futexes.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/futex.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index f3a3a07..bbf071f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -143,9 +143,8 @@
  *
  * Where (A) orders the waiters increment and the futex value read through
  * atomic operations (see hb_waiters_inc) and where (B) orders the write
- * to futex and the waiters read -- this is done by the barriers in
- * get_futex_key_refs(), through either ihold or atomic_inc, depending on the
- * futex type.
+ * to futex and the waiters read -- this is done by the barriers for both
+ * shared and private futexes in get_futex_key_refs().
  *
  * This yields the following case (where X:=waiters, Y:=futex):
  *
@@ -344,13 +343,20 @@ static void get_futex_key_refs(union futex_key *key)
 		futex_get_mm(key); /* implies MB (B) */
 		break;
 	default:
+		/*
+		 * Private futexes do not hold reference on an inode or
+		 * 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) */
 	}
 }
 
 /*
  * Drop a reference to the resource addressed by a key.
- * The hash bucket spinlock must not be held.
+ * The hash bucket spinlock must not be held. This is
+ * a no-op for private futexes, see comment in the get
+ * counterpart.
  */
 static void drop_futex_key_refs(union futex_key *key)
 {
-- 
1.8.4.5