[PATCH v3 06/12] locking/ww_mutex: Add waiters in stamp order

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

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

Add regular waiters in stamp order. Keep adding waiters that have no
context in FIFO order and take care not to starve them.

While adding our task as a waiter, back off if we detect that there is a
waiter with a lower stamp in front of us.

Make sure to call lock_contended even when we back off early.

For w/w mutexes, being first in the wait list is only stable when taking the
lock without a context. Therefore, the purpose of the first flag is split into
two: 'first' remains to indicate whether we want to spin optimistically, while
'handoff' indicates that we should be prepared to accept a handoff.

For w/w locking with a context, we always accept handoffs after the first
schedule(), to handle the following sequence of events:

1. Task #0 unlocks and hands off to Task #2 which is first in line
2. Task #1 adds itself in front of Task #2
3. Task #2 wakes up and must accept the handoff even though it is no longer
   first in line

v2:
- rein in the indentation of __ww_mutex_add_waiter a bit
- set contending_lock in __ww_mutex_add_waiter (Chris Wilson)

v3:
- split 'first' into 'first' and 'handoff' to avoid moving the trylock calls
  around so much
- scan the wait_list in reverse order in __ww_mutex_add_waiter

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>
---
 include/linux/mutex.h  |  3 ++
 kernel/locking/mutex.c | 97 +++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 91 insertions(+), 9 deletions(-)

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index b97870f..118a3b6 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -20,6 +20,8 @@
 #include <linux/osq_lock.h>
 #include <linux/debug_locks.h>
 
+struct ww_acquire_ctx;
+
 /*
  * Simple, straightforward mutexes with strict semantics:
  *
@@ -75,6 +77,7 @@ static inline struct task_struct *__mutex_owner(struct mutex *lock)
 struct mutex_waiter {
 	struct list_head	list;
 	struct task_struct	*task;
+	struct ww_acquire_ctx	*ww_ctx;
 #ifdef CONFIG_DEBUG_MUTEXES
 	void			*magic;
 #endif
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 282c6de..5b1ca20 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -620,6 +620,52 @@ __ww_mutex_lock_check_stamp(struct mutex *lock, struct ww_acquire_ctx *ctx)
 	return 0;
 }
 
+static inline int __sched
+__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;
+
+	if (!ww_ctx) {
+		list_add_tail(&waiter->list, &lock->wait_list);
+		return 0;
+	}
+
+	/*
+	 * Add the waiter before the first waiter with a higher stamp.
+	 * Waiters without a context are skipped to avoid starving
+	 * them.
+	 */
+	pos = &lock->wait_list;
+	list_for_each_entry_reverse(cur, &lock->wait_list, list) {
+		if (!cur->ww_ctx)
+			continue;
+
+		if (__ww_ctx_stamp_after(ww_ctx, cur->ww_ctx)) {
+			/* Back off immediately if necessary. */
+			if (ww_ctx->acquired > 0) {
+#ifdef CONFIG_DEBUG_MUTEXES
+				struct ww_mutex *ww;
+
+				ww = container_of(lock, struct ww_mutex, base);
+				DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
+				ww_ctx->contending_lock = ww;
+#endif
+				return -EDEADLK;
+			}
+
+			break;
+		}
+
+		pos = &cur->list;
+	}
+
+	list_add_tail(&waiter->list, pos);
+	return 0;
+}
+
 /*
  * Lock a mutex (possibly interruptible), slowpath:
  */
@@ -632,6 +678,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 	struct mutex_waiter waiter;
 	unsigned long flags;
 	bool first = false;
+	bool handoff = false;
 	struct ww_mutex *ww;
 	int ret;
 
@@ -665,15 +712,25 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 	debug_mutex_lock_common(lock, &waiter);
 	debug_mutex_add_waiter(lock, &waiter, task);
 
-	/* add waiting tasks to the end of the waitqueue (FIFO): */
-	list_add_tail(&waiter.list, &lock->wait_list);
+	lock_contended(&lock->dep_map, ip);
+
+	if (!use_ww_ctx) {
+		/* add waiting tasks to the end of the waitqueue (FIFO): */
+		list_add_tail(&waiter.list, &lock->wait_list);
+	} else {
+		/* Add in stamp order, waking up waiters that must back off. */
+		ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
+		if (ret)
+			goto err_early_backoff;
+
+		waiter.ww_ctx = ww_ctx;
+	}
+
 	waiter.task = task;
 
 	if (__mutex_waiter_is_first(lock, &waiter))
 		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
 
-	lock_contended(&lock->dep_map, ip);
-
 	set_task_state(task, state);
 	for (;;) {
 		/*
@@ -682,7 +739,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		 * before testing the error conditions to make sure we pick up
 		 * the handoff.
 		 */
-		if (__mutex_trylock(lock, first))
+		if (__mutex_trylock(lock, handoff))
 			goto acquired;
 
 		/*
@@ -711,13 +768,34 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		 * or we must see its unlock and acquire.
 		 */
 
-		if (!first && __mutex_waiter_is_first(lock, &waiter)) {
-			first = true;
+		if (use_ww_ctx && ww_ctx) {
+			/*
+			 * Always re-check whether we're in first position. We
+			 * don't want to spin if another task with a lower
+			 * stamp has taken our position.
+			 *
+			 * We also may have to set the handoff flag again, if
+			 * our position at the head was temporarily taken away.
+			 */
+			first = __mutex_waiter_is_first(lock, &waiter);
+
+			if (first)
+				__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
+
+			/*
+			 * Always be prepared to accept a handoff after the
+			 * first wait, because we may have been the first
+			 * waiter during unlock.
+			 */
+			handoff = true;
+		} else if (!first && __mutex_waiter_is_first(lock, &waiter)) {
+			first = handoff = true;
 			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
 		}
 
-		if ((first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true)) ||
-		     __mutex_trylock(lock, first))
+		if ((first &&
+		     mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true)) ||
+		    __mutex_trylock(lock, handoff))
 			break;
 
 		spin_lock_mutex(&lock->wait_lock, flags);
@@ -746,6 +824,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 err:
 	__set_task_state(task, TASK_RUNNING);
 	mutex_remove_waiter(lock, &waiter, task);
+err_early_backoff:
 	spin_unlock_mutex(&lock->wait_lock, flags);
 	debug_mutex_free_waiter(&waiter);
 	mutex_release(&lock->dep_map, 1, ip);
-- 
2.7.4

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