[PATCH 07/17] futex: Cleanup refcounting

[Henrik Austad <henrik@austad.us>]

From: Peter Zijlstra <peterz@infradead.org>

commit bf92cf3a5100f5a0d5f9834787b130159397cb22 upstream.

Add a put_pit_state() as counterpart for get_pi_state() so the refcounting
becomes consistent.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: juri.lelli@arm.com
Cc: bigeasy@linutronix.de
Cc: xlpang@redhat.com
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: jdesfossez@efficios.com
Cc: dvhart@infradead.org
Cc: bristot@redhat.com
Link: http://lkml.kernel.org/r/20170322104151.801778516@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Conflicts:
	kernel/futex.c
Tested-by:Henrik Austad <haustad@cisco.com>
---
 kernel/futex.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 52e3678..9d7d462 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -799,7 +799,7 @@ static int refill_pi_state_cache(void)
 	return 0;
 }
 
-static struct futex_pi_state * alloc_pi_state(void)
+static struct futex_pi_state *alloc_pi_state(void)
 {
 	struct futex_pi_state *pi_state = current->pi_state_cache;
 
@@ -809,6 +809,11 @@ static struct futex_pi_state * alloc_pi_state(void)
 	return pi_state;
 }
 
+static void get_pi_state(struct futex_pi_state *pi_state)
+{
+	WARN_ON_ONCE(!atomic_inc_not_zero(&pi_state->refcount));
+}
+
 /*
  * Must be called with the hb lock held.
  */
@@ -850,7 +855,7 @@ static void free_pi_state(struct futex_pi_state *pi_state)
  * Look up the task based on what TID userspace gave us.
  * We dont trust it.
  */
-static struct task_struct * futex_find_get_task(pid_t pid)
+static struct task_struct *futex_find_get_task(pid_t pid)
 {
 	struct task_struct *p;
 
@@ -1097,7 +1102,7 @@ static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
 		goto out_einval;
 
 out_attach:
-	atomic_inc(&pi_state->refcount);
+	get_pi_state(pi_state);
 	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
 	*ps = pi_state;
 	return 0;
@@ -2019,8 +2024,12 @@ retry_private:
 		 * of requeue_pi if we couldn't acquire the lock atomically.
 		 */
 		if (requeue_pi) {
-			/* Prepare the waiter to take the rt_mutex. */
-			atomic_inc(&pi_state->refcount);
+			/*
+			 * Prepare the waiter to take the rt_mutex. Take a
+			 * refcount on the pi_state and store the pointer in
+			 * the futex_q object of the waiter.
+			 */
+			get_pi_state(pi_state);
 			this->pi_state = pi_state;
 			ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
 							this->rt_waiter,
-- 
2.7.4