futex: comment requeue key reference semantics

We've tripped over the futex_requeue drop_count refering to key2
instead of key1.  The code is actually correct, but is non-intuitive.
This patch adds an explicit comment explaining the requeue.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 6 insertions, 1 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 6b50a024bca2..eef8cd26b5e5 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -883,7 +883,12 @@ retry_private:
 out_unlock:
         double_unlock_hb(hb1, hb2);
 
-        /* drop_futex_key_refs() must be called outside the spinlocks. */
+        /*
+         * drop_futex_key_refs() must be called outside the spinlocks. During
+         * the requeue we moved futex_q's from the hash bucket at key1 to the
+         * one at key2 and updated their key pointer.  We no longer need to
+         * hold the references to key1.
+         */
         while (--drop_count >= 0)
                 drop_futex_key_refs(&key1);