locking/ww_mutex: Notify waiters that have to back off while adding tasks to wait list

While adding our task as a waiter, detect if another task should back off
because of us.

With this patch, we establish the invariant that the wait list contains
at most one (sleeping) waiter with ww_ctx->acquired > 0, and this waiter
will be the first waiter with a context.

Since only waiters with ww_ctx->acquired > 0 have to back off, this allows
us to be much more economical with wakeups.
Signed-off-by: Nicolai Hähnle <Nicolai.Haehnle@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maarten Lankhorst <dev@mblankhorst.nl>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dri-devel@lists.freedesktop.org
Link: http://lkml.kernel.org/r/1482346000-9927-8-git-send-email-nhaehnle@gmail.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/mutex.c

@@ -596,23 +596,34 @@ void __sched ww_mutex_unlock(struct ww_mutex *lock)
 EXPORT_SYMBOL(ww_mutex_unlock);
 
 static inline int __sched
-__ww_mutex_lock_check_stamp(struct mutex *lock, struct ww_acquire_ctx *ctx)
+__ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
+			    struct ww_acquire_ctx *ctx)
 {
 	struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
 	struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);
+	struct mutex_waiter *cur;
+
+	if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
+		goto deadlock;
+
+	/*
+	 * If there is a waiter in front of us that has a context, then its
+	 * stamp is earlier than ours and we must back off.
+	 */
+	cur = waiter;
+	list_for_each_entry_continue_reverse(cur, &lock->wait_list, list) {
+		if (cur->ww_ctx)
+			goto deadlock;
+	}
 
-	if (!hold_ctx)
 	return 0;
 
-	if (__ww_ctx_stamp_after(ctx, hold_ctx)) {
+deadlock:
 #ifdef CONFIG_DEBUG_MUTEXES
 	DEBUG_LOCKS_WARN_ON(ctx->contending_lock);
 	ctx->contending_lock = ww;
 #endif
 	return -EDEADLK;
-	}
-
-	return 0;
 }
 
 static inline int __sched
@@ -655,6 +666,15 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
 		}
 
 		pos = &cur->list;
+
+		/*
+		 * Wake up the waiter so that it gets a chance to back
+		 * off.
+		 */
+		if (cur->ww_ctx->acquired > 0) {
+			debug_mutex_wake_waiter(lock, cur);
+			wake_up_process(cur->task);
+		}
 	}
 
 	list_add_tail(&waiter->list, pos);
@@ -746,7 +766,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		}
 
 		if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
-			ret = __ww_mutex_lock_check_stamp(lock, ww_ctx);
+			ret = __ww_mutex_lock_check_stamp(lock, &waiter, ww_ctx);
 			if (ret)
 				goto err;
 		}