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-rw-r--r--kernel/futex.c45
1 files changed, 28 insertions, 17 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index be3bff2315ff..a0514e01c3eb 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1452,11 +1452,7 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
return;
- /*
- * Queue the task for later wakeup for after we've released
- * the hb->lock. wake_q_add() grabs reference to p.
- */
- wake_q_add(wake_q, p);
+ get_task_struct(p);
__unqueue_futex(q);
/*
* The waiting task can free the futex_q as soon as q->lock_ptr = NULL
@@ -1466,6 +1462,13 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
* plist_del in __unqueue_futex().
*/
smp_store_release(&q->lock_ptr, NULL);
+
+ /*
+ * Queue the task for later wakeup for after we've released
+ * the hb->lock. wake_q_add() grabs reference to p.
+ */
+ wake_q_add(wake_q, p);
+ put_task_struct(p);
}
/*
@@ -2218,11 +2221,11 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
* decrement the counter at queue_unlock() when some error has
* occurred and we don't end up adding the task to the list.
*/
- hb_waiters_inc(hb);
+ hb_waiters_inc(hb); /* implies smp_mb(); (A) */
q->lock_ptr = &hb->lock;
- spin_lock(&hb->lock); /* implies smp_mb(); (A) */
+ spin_lock(&hb->lock);
return hb;
}
@@ -2858,35 +2861,39 @@ retry_private:
* and BUG when futex_unlock_pi() interleaves with this.
*
* Therefore acquire wait_lock while holding hb->lock, but drop the
- * latter before calling rt_mutex_start_proxy_lock(). This still fully
- * serializes against futex_unlock_pi() as that does the exact same
- * lock handoff sequence.
+ * latter before calling __rt_mutex_start_proxy_lock(). This
+ * interleaves with futex_unlock_pi() -- which does a similar lock
+ * handoff -- such that the latter can observe the futex_q::pi_state
+ * before __rt_mutex_start_proxy_lock() is done.
*/
raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
spin_unlock(q.lock_ptr);
+ /*
+ * __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
+ * such that futex_unlock_pi() is guaranteed to observe the waiter when
+ * it sees the futex_q::pi_state.
+ */
ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);
if (ret) {
if (ret == 1)
ret = 0;
-
- spin_lock(q.lock_ptr);
- goto no_block;
+ goto cleanup;
}
-
if (unlikely(to))
hrtimer_start_expires(&to->timer, HRTIMER_MODE_ABS);
ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);
+cleanup:
spin_lock(q.lock_ptr);
/*
- * If we failed to acquire the lock (signal/timeout), we must
+ * If we failed to acquire the lock (deadlock/signal/timeout), we must
* first acquire the hb->lock before removing the lock from the
- * rt_mutex waitqueue, such that we can keep the hb and rt_mutex
- * wait lists consistent.
+ * rt_mutex waitqueue, such that we can keep the hb and rt_mutex wait
+ * lists consistent.
*
* In particular; it is important that futex_unlock_pi() can not
* observe this inconsistency.
@@ -3010,6 +3017,10 @@ retry:
* there is no point where we hold neither; and therefore
* wake_futex_pi() must observe a state consistent with what we
* observed.
+ *
+ * In particular; this forces __rt_mutex_start_proxy() to
+ * complete such that we're guaranteed to observe the
+ * rt_waiter. Also see the WARN in wake_futex_pi().
*/
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);