futex: Document pi_state refcounting in requeue code
Documentation of the pi_state refcounting in the requeue code is non
existent. Add it.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <darren@dvhart.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Bhuvanesh_Surachari@mentor.com
Cc: Andy Lowe <Andy_Lowe@mentor.com>
Link: http://lkml.kernel.org/r/20151219200607.335938312@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
diff --git a/kernel/futex.c b/kernel/futex.c
index f1581ff..20c4683 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1709,27 +1709,31 @@
* exist yet, look it up one more time to ensure we have a
* reference to it. If the lock was taken, ret contains the
* vpid of the top waiter task.
+ * If the lock was not taken, we have pi_state and an initial
+ * refcount on it. In case of an error we have nothing.
*/
if (ret > 0) {
WARN_ON(pi_state);
drop_count++;
task_count++;
/*
- * If we acquired the lock, then the user
- * space value of uaddr2 should be vpid. It
- * cannot be changed by the top waiter as it
- * is blocked on hb2 lock if it tries to do
- * so. If something fiddled with it behind our
- * back the pi state lookup might unearth
- * it. So we rather use the known value than
- * rereading and handing potential crap to
- * lookup_pi_state.
+ * If we acquired the lock, then the user space value
+ * of uaddr2 should be vpid. It cannot be changed by
+ * the top waiter as it is blocked on hb2 lock if it
+ * tries to do so. If something fiddled with it behind
+ * our back the pi state lookup might unearth it. So
+ * we rather use the known value than rereading and
+ * handing potential crap to lookup_pi_state.
+ *
+ * If that call succeeds then we have pi_state and an
+ * initial refcount on it.
*/
ret = lookup_pi_state(ret, hb2, &key2, &pi_state);
}
switch (ret) {
case 0:
+ /* We hold a reference on the pi state. */
break;
case -EFAULT:
put_pi_state(pi_state);
@@ -1804,19 +1808,37 @@
* of requeue_pi if we couldn't acquire the lock atomically.
*/
if (requeue_pi) {
- /* Prepare the waiter to take the rt_mutex. */
+ /*
+ * 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.
+ */
atomic_inc(&pi_state->refcount);
this->pi_state = pi_state;
ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
this->rt_waiter,
this->task);
if (ret == 1) {
- /* We got the lock. */
+ /*
+ * We got the lock. We do neither drop the
+ * refcount on pi_state nor clear
+ * this->pi_state because the waiter needs the
+ * pi_state for cleaning up the user space
+ * value. It will drop the refcount after
+ * doing so.
+ */
requeue_pi_wake_futex(this, &key2, hb2);
drop_count++;
continue;
} else if (ret) {
- /* -EDEADLK */
+ /*
+ * rt_mutex_start_proxy_lock() detected a
+ * potential deadlock when we tried to queue
+ * that waiter. Drop the pi_state reference
+ * which we took above and remove the pointer
+ * to the state from the waiters futex_q
+ * object.
+ */
this->pi_state = NULL;
put_pi_state(pi_state);
goto out_unlock;
@@ -1827,6 +1849,11 @@
}
out_unlock:
+ /*
+ * We took an extra initial reference to the pi_state either
+ * in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
+ * need to drop it here again.
+ */
put_pi_state(pi_state);
double_unlock_hb(hb1, hb2);
wake_up_q(&wake_q);