kernel/futex: Kill rt_mutex_next_owner()

Update wake_futex_pi() and kill the call altogether. This is possible because:

(i) The case of fixup_owner() in which the pi_mutex was stolen from the
signaled enqueued top-waiter which fails to trylock and doesn't see a
current owner of the rtmutex but needs to acknowledge an non-enqueued
higher priority waiter, which is the other alternative. This used to be
handled by rt_mutex_next_owner(), which guaranteed fixup_pi_state_owner('newowner')
never to be nil. Nowadays the logic is handled by an EAGAIN loop, without
the need of rt_mutex_next_owner(). Specifically:

    c1e2f0ea (futex: Avoid violating the 10th rule of futex)
    9f5d1c33 (futex: Handle transient "ownerless" rtmutex state correctly)

(ii) rt_mutex_next_owner() and rt_mutex_top_waiter() are semantically
equivalent, as of:

    c28d62cf (locking/rtmutex: Handle non enqueued waiters gracefully in remove_waiter())

So instead of keeping the call around, just use the good ole rt_mutex_top_waiter().
No change in semantics.
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210226175029.50335-1-dave@stgolabs.net

kernel/futex.c

@@ -1494,13 +1494,14 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
 static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state)
 {
 	u32 curval, newval;
+	struct rt_mutex_waiter *top_waiter;
 	struct task_struct *new_owner;
 	bool postunlock = false;
 	DEFINE_WAKE_Q(wake_q);
 	int ret = 0;
 
-	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
-	if (WARN_ON_ONCE(!new_owner)) {
+	top_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);
+	if (WARN_ON_ONCE(!top_waiter)) {
 		/*
 		 * As per the comment in futex_unlock_pi() this should not happen.
 		 *
@@ -1513,6 +1514,8 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_
 		goto out_unlock;
 	}
 
+	new_owner = top_waiter->task;
+
 	/*
 	 * We pass it to the next owner. The WAITERS bit is always kept
 	 * enabled while there is PI state around. We cleanup the owner

kernel/locking/rtmutex.c

@@ -1792,26 +1792,6 @@ int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
 	return ret;
 }
 
-/**
- * rt_mutex_next_owner - return the next owner of the lock
- *
- * @lock: the rt lock query
- *
- * Returns the next owner of the lock or NULL
- *
- * Caller has to serialize against other accessors to the lock
- * itself.
- *
- * Special API call for PI-futex support
- */
-struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock)
-{
-	if (!rt_mutex_has_waiters(lock))
-		return NULL;
-
-	return rt_mutex_top_waiter(lock)->task;
-}
-
 /**
  * rt_mutex_wait_proxy_lock() - Wait for lock acquisition
  * @lock:		the rt_mutex we were woken on

kernel/locking/rtmutex_common.h

@@ -130,7 +130,6 @@ enum rtmutex_chainwalk {
 /*
  * PI-futex support (proxy locking functions, etc.):
  */
-extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock);
 extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
 				       struct task_struct *proxy_owner);
 extern void rt_mutex_proxy_unlock(struct rt_mutex *lock);