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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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      Last update: 2009-08-08 17:33    [W:0.034 / U:174.604 seconds]
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