[PATCH 2/3] futex: avoid double wake up in futex_wake() on -RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

futex_wake() wakes the waiter while holding the hb->lock. This leads to
a similar double wake up on -RT if the waiter has a higher priority than
the process perfroming the wake up.

This patch allocates space for one task and lets futex_wake() to wake up
the task once the hb->lock is dropped. If futex_wake() is used to wake
up more than once task then the behaviour remains unchanged. With one
waiter however we avoid the double wake up on -RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/futex.c | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index b38abe3573a8..f3e633e16973 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1088,16 +1088,12 @@ static void __unqueue_futex(struct futex_q *q)
 	hb_waiters_dec(hb);
 }
 
-/*
- * The hash bucket lock must be held when this is called.
- * Afterwards, the futex_q must not be accessed.
- */
-static void wake_futex(struct futex_q *q)
+static struct task_struct *__wake_futex(struct futex_q *q)
 {
 	struct task_struct *p = q->task;
 
 	if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
-		return;
+		return NULL;
 
 	/*
 	 * We set q->lock_ptr = NULL _before_ we wake up the task. If
@@ -1117,6 +1113,19 @@ static void wake_futex(struct futex_q *q)
 	 */
 	smp_wmb();
 	q->lock_ptr = NULL;
+	return p;
+}
+
+/*
+ * The hash bucket lock must be held when this is called.
+ * Afterwards, the futex_q must not be accessed.
+ */
+static void wake_futex(struct futex_q *q)
+{
+	struct task_struct *p = __wake_futex(q);
+
+	if (!p)
+		return;
 
 	wake_up_state(p, TASK_NORMAL);
 	put_task_struct(p);
@@ -1228,6 +1237,7 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
 	struct futex_hash_bucket *hb;
 	struct futex_q *this, *next;
 	union futex_key key = FUTEX_KEY_INIT;
+	struct task_struct *waiter = NULL;
 	int ret;
 
 	if (!bitset)
@@ -1256,14 +1266,23 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
 			if (!(this->bitset & bitset))
 				continue;
 
-			wake_futex(this);
+			if (nr_wake == 1)
+				waiter = __wake_futex(this);
+			else
+				wake_futex(this);
 			if (++ret >= nr_wake)
 				break;
 		}
 	}
 
 	spin_unlock(&hb->lock);
+
 out_put_key:
+	if (waiter) {
+		wake_up_state(waiter, TASK_NORMAL);
+		put_task_struct(waiter);
+	}
+
 	put_futex_key(&key);
 out:
 	return ret;
-- 
2.1.4