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Subject[tip:core/urgent] futex: Update woken requeued futex_q lock_ptr
Commit-ID:  00235fe25eba6d3a13f3349b2e3a2d94b699a414
Author: Darren Hart <>
AuthorDate: Wed, 5 Aug 2009 15:02:20 -0700
Committer: Ingo Molnar <>
CommitDate: Sat, 8 Aug 2009 17:21:49 +0200

futex: Update woken requeued futex_q lock_ptr

futex_requeue() can acquire the lock on behalf of a waiter
during the requeue loop in the event of a lock steal or owner
died. 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 found 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.

The astute reviewer will note that still exists a race between
the time futex_requeue() releases hb2->lock() and the time when
futex_wait_requeue_pi() acquires it. During this time the
pi_state and the futex uaddr are not in sync with the rt_mutex
ownership. This patch closes the window to the point where my
tests now pass, but we still need to address it.

Signed-off-by: Darren Hart <>
Cc: Peter Zijlstra <>
Cc: Steven Rostedt <>
Cc: Dinakar Guniguntala <>
Cc: John Stultz <>
Cc: John Kacur <>
Cc: <>
LKML-Reference: <>
Signed-off-by: Ingo Molnar <>

kernel/futex.c | 13 +++++++++----
1 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 0672ff8..57f5a80 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1010,19 +1010,24 @@ 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 the q->lock_ptr 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)
q->key = *key;
+ q->lock_ptr = &hb->lock;

plist_del(&q->list, &q->list.plist);
@@ -1088,7 +1093,7 @@ static int futex_proxy_trylock_atomic(u32 __user *pifutex,
ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
if (ret == 1)
- requeue_pi_wake_futex(top_waiter, key2);
+ requeue_pi_wake_futex(top_waiter, key2, hb2);

return ret;
@@ -1273,7 +1278,7 @@ 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);
} else if (ret) {
/* -EDEADLK */

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