[tip:core/urgent] futex: Update woken requeued futex_q lock_ptr

[tip-bot for Darren Hart]

Commit-ID:  4047446de8fa83d8d5922e9448eb0cbb7ac3f475
Gitweb:     http://git.kernel.org/tip/4047446de8fa83d8d5922e9448eb0cbb7ac3f475
Author:     Darren Hart <dvhltc@xxxxxxxxxx>
AuthorDate: Wed, 5 Aug 2009 15:02:20 -0700
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Sun, 9 Aug 2009 22:20:07 +0200

futex: Update woken requeued futex_q lock_ptr

futex_requeue() can acquire the lock on behalf of a waiter early on
or during the requeue loop if it is uncontended or in the event of a
lock steal or owner died. On wakeup, the waiter (in
futex_wait_requeue_pi()) cleans up the pi_state owner using the
lock_ptr to protect against concurrent access to the pi_state. The
pi_state is hung off futex_q's on the requeue target futex hash
bucket so the lock_ptr needs to be updated accordingly.

The problem manifested by triggering the WARN_ON in
lookup_pi_state() about the pid != pi_state->owner->pid.  With this
patch, the pi_state is properly guarded against concurrent access
via the requeue target hb lock.

The astute reviewer may notice that there is a window of time
between when futex_requeue() unlocks the hb locks and when
futex_wait_requeue_pi() will acquire hb2->lock.  During this time
the pi_state and uval are not in sync with the underlying rtmutex
owner (but the uval does indicate there are waiters, so no atomic
changes will occur in userspace).  However, this is not a problem.
Should a contending thread enter lookup_pi_state() and acquire
hb2->lock before the ownership is fixed up, it will find the
pi_state hung off a waiter's (possibly the pending owner's) futex_q
and block on the rtmutex.  Once futex_wait_requeue_pi() fixes up the
owner, it will also move the pi_state from the old owner's
task->pi_state_list to its own.

Signed-off-by: Darren Hart <dvhltc@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Dinakar Guniguntala <dino@xxxxxxxxxx>
Cc: John Stultz <johnstul@xxxxxxxxxx>
Cc: John Kacur <jkacur@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
LKML-Reference: <4A7A016C.1090002@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>


---
 kernel/futex.c |   17 +++++++++++++----
 1 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 0672ff8..ca99305 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1010,15 +1010,19 @@ void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
  * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
  * q:	the futex_q
  * key:	the key of the requeue target futex
+ * hb:  the hash_bucket of the requeue target futex
  *
  * During futex_requeue, with requeue_pi=1, it is possible to acquire the
  * target futex if it is uncontended or via a lock steal.  Set the futex_q key
  * to the requeue target futex so the waiter can detect the wakeup on the right
  * futex, but remove it from the hb and NULL the rt_waiter so it can detect
- * atomic lock acquisition.  Must be called with the q->lock_ptr held.
+ * atomic lock acquisition.  Set the q->lock_ptr to the requeue target hb->lock
+ * to protect access to the pi_state to fixup the owner later.  Must be called
+ * with both q->lock_ptr and hb->lock held.
  */
 static inline
-void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key)
+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);
@@ -1030,6 +1034,11 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key)
 	WARN_ON(!q->rt_waiter);
 	q->rt_waiter = NULL;
 
+	q->lock_ptr = &hb->lock;
+#ifdef CONFIG_DEBUG_PI_LIST
+	q->list.plist.slock = &hb->lock;
+#endif
+
 	wake_up_state(q->task, TASK_NORMAL);
 }
 
@@ -1088,7 +1097,7 @@ static int futex_proxy_trylock_atomic(u32 __user *pifutex,
 	ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
 				   set_waiters);
 	if (ret == 1)
-		requeue_pi_wake_futex(top_waiter, key2);
+		requeue_pi_wake_futex(top_waiter, key2, hb2);
 
 	return ret;
 }
@@ -1273,7 +1282,7 @@ retry_private:
 							this->task, 1);
 			if (ret == 1) {
 				/* We got the lock. */
-				requeue_pi_wake_futex(this, &key2);
+				requeue_pi_wake_futex(this, &key2, hb2);
 				continue;
 			} else if (ret) {
 				/* -EDEADLK */
--
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