[RFC][PATCH -v3 10/10] locking/mutex: Implement alternative HANDOFF

[Peter Zijlstra]

As mentioned in a previous patch, its possible to implement the
handoff logic differently, avoiding the issue where we 'leak' a HADOFF
flag.

This patch does so, just to show what it looks like; I'm not at all
convinced this is worth it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/locking/mutex.c |   34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -71,6 +71,32 @@ static inline unsigned long __owner_flag
 	return owner & MUTEX_FLAGS;
 }
 
+static inline bool __mutex_req_handoff(struct mutex *lock)
+{
+	unsigned long owner, curr = (unsigned long)current;
+
+	owner = atomic_long_read(&lock->owner);
+	for (;;) {
+		unsigned long old, new;
+
+		if (owner & MUTEX_FLAG_HANDOFF)
+			return false;
+
+		if (__owner_task(owner))
+			new = owner | MUTEX_FLAG_HANDOFF;
+		else
+			new = curr | __owner_flags(owner);
+
+		old = atomic_long_cmpxchg_acquire(&lock->owner, owner, new);
+		if (old == owner)
+			break;
+
+		owner = old;
+	}
+
+	return !__owner_task(owner);
+}
+
 /*
  * Actual trylock that will work on any unlocked state.
  *
@@ -609,7 +635,6 @@ __mutex_lock_common(struct mutex *lock,
 	struct task_struct *task = current;
 	struct mutex_waiter waiter;
 	unsigned long flags;
-	bool first = false;
 	struct ww_mutex *ww;
 	int ret;
 
@@ -676,8 +701,9 @@ __mutex_lock_common(struct mutex *lock,
 		schedule_preempt_disabled();
 
 		if (__mutex_waiter_is_first(lock, &waiter)) {
-			first = true;
-			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
+			if (__mutex_req_handoff(lock))
+				break;
+
 			if (mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true))
 				break;
 		}
@@ -700,8 +726,6 @@ __mutex_lock_common(struct mutex *lock,
 	mutex_remove_waiter(lock, &waiter, task);
 	if (likely(list_empty(&lock->wait_list)))
 		__mutex_clear_flag(lock, MUTEX_FLAGS);
-	else if (first && (atomic_long_read(&lock->owner) & MUTEX_FLAG_HANDOFF))
-		__mutex_clear_flag(lock, MUTEX_FLAG_HANDOFF);
 
 	debug_mutex_free_waiter(&waiter);