locking/ww_mutex: Abstract out waiter enqueueing

The upcoming rtmutex based ww_mutex needs a different handling for
enqueueing a waiter. Split it out into a helper function.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211304.566318143@linutronix.de

kernel/locking/ww_mutex.h

@@ -44,6 +44,15 @@ __ww_waiter_last(struct mutex *lock)
 	return w;
 }
 
+static inline void
+__ww_waiter_add(struct mutex *lock, struct mutex_waiter *waiter, struct mutex_waiter *pos)
+{
+	struct list_head *p = &lock->wait_list;
+	if (pos)
+		p = &pos->list;
+	__mutex_add_waiter(lock, waiter, p);
+}
+
 /*
  * Wait-Die:
  *   The newer transactions are killed when:
@@ -337,12 +346,11 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
 		      struct mutex *lock,
 		      struct ww_acquire_ctx *ww_ctx)
 {
-	struct mutex_waiter *cur;
-	struct list_head *pos;
+	struct mutex_waiter *cur, *pos = NULL;
 	bool is_wait_die;
 
 	if (!ww_ctx) {
-		__mutex_add_waiter(lock, waiter, &lock->wait_list);
+		__ww_waiter_add(lock, waiter, NULL);
 		return 0;
 	}
 
@@ -355,7 +363,6 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
 	 * never die here, but they are sorted in stamp order and
 	 * may wound the lock holder.
 	 */
-	pos = &lock->wait_list;
 	for (cur = __ww_waiter_last(lock); cur;
 	     cur = __ww_waiter_prev(lock, cur)) {
 
@@ -378,13 +385,13 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
 			break;
 		}
 
-		pos = &cur->list;
+		pos = cur;
 
 		/* Wait-Die: ensure younger waiters die. */
 		__ww_mutex_die(lock, cur, ww_ctx);
 	}
 
-	__mutex_add_waiter(lock, waiter, pos);
+	__ww_waiter_add(lock, waiter, pos);
 
 	/*
 	 * Wound-Wait: if we're blocking on a mutex owned by a younger context,