[PATCH v2 11/11] [rfc] locking/ww_mutex: Always spin optimistically for the first waiter

["Nicolai Hähnle" <nhaehnle@gmail.com>]

From: Nicolai Hähnle <Nicolai.Haehnle@amd.com>

Check the current owner's context once against our stamp. If our stamp is
lower, we continue to spin optimistically instead of backing off.

This is correct with respect to deadlock detection because while the
(owner, ww_ctx) pair may re-appear if the owner task manages to unlock
and re-acquire the lock while we're spinning, the context could only have
been re-initialized with an even higher stamp. We also still detect when
we have to back off for other waiters that join the list while we're
spinning.

But taking the wait_lock in mutex_spin_on_owner feels iffy, even if it is
done only once.

Median timings taken of a contention-heavy GPU workload:

Before:
real    0m53.086s
user    0m7.360s
sys     1m46.204s

After:
real    0m52.577s
user    0m7.544s
sys     1m49.200s

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Maarten Lankhorst <dev@mblankhorst.nl>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Nicolai Hähnle <Nicolai.Haehnle@amd.com>
---
 kernel/locking/mutex.c | 35 ++++++++++++++++++++++++++++++++---
 1 file changed, 32 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 38d173c..9216239 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -378,6 +378,28 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 			 struct mutex_waiter *waiter)
 {
 	bool ret = true;
+	struct ww_acquire_ctx *owner_ww_ctx = NULL;
+
+	if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
+		struct ww_mutex *ww;
+		unsigned long flags;
+
+		ww = container_of(lock, struct ww_mutex, base);
+
+		/*
+		 * Check the stamp of the current owner once. This allows us
+		 * to spin optimistically in the case where the current owner
+		 * has a higher stamp than us.
+		 */
+		spin_lock_mutex(&lock->wait_lock, flags);
+		owner_ww_ctx = ww->ctx;
+		if (owner_ww_ctx &&
+		    __ww_mutex_stamp_after(ww_ctx, owner_ww_ctx)) {
+			spin_unlock_mutex(&lock->wait_lock, flags);
+			return false;
+		}
+		spin_unlock_mutex(&lock->wait_lock, flags);
+	}
 
 	rcu_read_lock();
 	while (__mutex_owner(lock) == owner) {
@@ -414,9 +436,16 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 			 * Check this in every inner iteration because we may
 			 * be racing against another thread's ww_mutex_lock.
 			 */
-			if (ww_ctx->acquired > 0 && READ_ONCE(ww->ctx)) {
-				ret = false;
-				break;
+			if (ww_ctx->acquired > 0) {
+				struct ww_acquire_ctx *current_ctx;
+
+				current_ctx = READ_ONCE(ww->ctx);
+
+				if (current_ctx &&
+				    current_ctx != owner_ww_ctx) {
+					ret = false;
+					break;
+				}
 			}
 
 			/*
-- 
2.7.4