[PATCH] futex: Move drop_futex_key_refs out of spinlock'ed region

[PATCH] futex: Move drop_futex_key_refs out of spinlock'ed region

[Darren Hart]

>From edd4dea2dfb706d34a42255f424781f9daa79f01 Mon Sep 17 00:00:00 2001
From: Helge Bahmann <hcb@chaoticmind.net>
Date: Mon, 12 Oct 2009 11:31:35 +0200
Subject: [PATCH] Move drop_futex_key_refs out of spinlock'ed region

When requeuing tasks from one futex to another, the reference held
by the requeued task to the original futex location needs to
be dropped eventually. Dropping the reference may ultimately lead
to a call to "iput_final" and subsequently call into filesystem-
specific code. It is therefore safer to defer this drop operation
until after the futex_hash_bucket spinlock has been dropped.

Authored-by: Helge Bahmann <hcb@chaoticmind.net>
Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
CC: stable@kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
CC: Eric Dumazet <eric.dumazet@gmail.com>
CC: Dinakar Guniguntala <dino@in.ibm.com>
CC: John Stultz <johnstul@us.ibm.com>
CC: Sven-Thorsten Dietrich <sdietrich@novell.com>
CC: John Kacur <jkacur@redhat.com>


---
 kernel/futex.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 4949d33..f49bf07 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1028,7 +1028,6 @@ static inline
 void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
 			   struct futex_hash_bucket *hb)
 {
-	drop_futex_key_refs(&q->key);
 	get_futex_key_refs(key);
 	q->key = *key;
 
@@ -1226,6 +1225,7 @@ retry_private:
 		 */
 		if (ret == 1) {
 			WARN_ON(pi_state);
+			drop_count++;
 			task_count++;
 			ret = get_futex_value_locked(&curval2, uaddr2);
 			if (!ret)
@@ -1304,6 +1304,7 @@ retry_private:
 			if (ret == 1) {
 				/* We got the lock. */
 				requeue_pi_wake_futex(this, &key2, hb2);
+				drop_count++;
 				continue;
 			} else if (ret) {
 				/* -EDEADLK */
-- 
1.5.6.5


-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team

[tip:core/urgent] futex: Move drop_futex_key_refs out of spinlock'ed region

[tip-bot for Darren Hart]

Commit-ID:  89061d3d58e1f0742139605dc6a7950aa1ecc019
Gitweb:     http://git.kernel.org/tip/89061d3d58e1f0742139605dc6a7950aa1ecc019
Author:     Darren Hart <dvhltc@us.ibm.com>
AuthorDate: Thu, 15 Oct 2009 15:30:48 -0700
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 16 Oct 2009 10:19:18 +0200

futex: Move drop_futex_key_refs out of spinlock'ed region

When requeuing tasks from one futex to another, the reference held
by the requeued task to the original futex location needs to be
dropped eventually.

Dropping the reference may ultimately lead to a call to
"iput_final" and subsequently call into filesystem- specific code -
which may be non-atomic.

It is therefore safer to defer this drop operation until after the
futex_hash_bucket spinlock has been dropped.

Originally-From: Helge Bahmann <hcb@chaoticmind.net>
Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: <stable@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Dinakar Guniguntala <dino@in.ibm.com>
Cc: John Stultz <johnstul@linux.vnet.ibm.com>
Cc: Sven-Thorsten Dietrich <sdietrich@novell.com>
Cc: John Kacur <jkacur@redhat.com>
LKML-Reference: <4AD7A298.5040802@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/futex.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 06938e5..642f3bb 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1029,7 +1029,6 @@ static inline
 void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
 			   struct futex_hash_bucket *hb)
 {
-	drop_futex_key_refs(&q->key);
 	get_futex_key_refs(key);
 	q->key = *key;
 
@@ -1227,6 +1226,7 @@ retry_private:
 		 */
 		if (ret == 1) {
 			WARN_ON(pi_state);
+			drop_count++;
 			task_count++;
 			ret = get_futex_value_locked(&curval2, uaddr2);
 			if (!ret)
@@ -1305,6 +1305,7 @@ retry_private:
 			if (ret == 1) {
 				/* We got the lock. */
 				requeue_pi_wake_futex(this, &key2, hb2);
+				drop_count++;
 				continue;
 			} else if (ret) {
 				/* -EDEADLK */