From: François Tigeot <ftigeot@wolfpond.org>
Date: Thu, 24 Dec 2015 08:56:57 +0000 (+0100)
Subject: drm: Fix ww mutex crtc locking
X-Git-Tag: v4.6.0rc~1131
X-Git-Url: https://gitweb.dragonflybsd.org/~tuxillo/dragonfly.git/commitdiff_plain/4a1f0d757dc89c9bbe3a5b0e16c28eb180939912

drm: Fix ww mutex crtc locking

* The original code had many problems:
  - the ctx wasn't available to drm_modeset_unlock().
  - 'struct list_head' was used for both the list head AND the list entry
  It's like it was coded to create maximum confusion on top of maximum pain

* drm_modeset_acquire_ctx structure needs a second list which we add
  kmalloc()'d entries to to track the extra locks in the EALREADY case

* Replace the reference from the modeset_lock structure with a pointer
  to an intermediate (kmalloc'd) structure that references both and turn
  the list entry in the lock into a list head

Submitted-by: Matthew Dillon <dillon@apollo.backplane.com>
---

diff --git a/sys/dev/drm/drm_modeset_lock.c b/sys/dev/drm/drm_modeset_lock.c
index 0a71b8f3c5..ddb9f8e06c 100644
--- a/sys/dev/drm/drm_modeset_lock.c
+++ b/sys/dev/drm/drm_modeset_lock.c
@@ -271,12 +271,12 @@ void drm_modeset_drop_locks(struct drm_modeset_acquire_ctx *ctx)
 {
 	WARN_ON(ctx->contended);
 	while (!list_empty(&ctx->locked)) {
-		struct drm_modeset_lock *lock;
+		struct drm_modeset_lock_info *info;
 
-		lock = list_first_entry(&ctx->locked,
-				struct drm_modeset_lock, head);
+		info = list_first_entry(&ctx->locked,
+				struct drm_modeset_lock_info, ctx_entry);
 
-		drm_modeset_unlock(lock);
+		drm_modeset_unlock(info->lock);
 	}
 }
 EXPORT_SYMBOL(drm_modeset_drop_locks);
@@ -299,10 +299,7 @@ static inline int modeset_lock(struct drm_modeset_lock *lock,
 	} else {
 		ret = ww_mutex_lock(&lock->mutex, &ctx->ww_ctx);
 	}
-	if (!ret) {
-		WARN_ON(!list_empty(&lock->head));
-		list_add(&lock->head, &ctx->locked);
-	} else if (ret == -EALREADY) {
+	if (ret == -EALREADY) {
 		/* we already hold the lock.. this is fine.  For atomic
 		 * we will need to be able to drm_modeset_lock() things
 		 * without having to keep track of what is already locked
@@ -312,6 +309,17 @@ static inline int modeset_lock(struct drm_modeset_lock *lock,
 	} else if (ret == -EDEADLK) {
 		ctx->contended = lock;
 	}
+	if (ret == 0) {
+		struct drm_modeset_lock_info *info;
+
+		info = kzalloc(sizeof(*info), GFP_KERNEL);
+		INIT_LIST_HEAD(&info->ctx_entry);
+		INIT_LIST_HEAD(&info->lock_entry);
+		info->lock = lock;
+		info->ctx = ctx;
+		list_add(&info->ctx_entry, &ctx->locked);
+		list_add(&info->lock_entry, &lock->locked);
+	}
 
 	return ret;
 }
@@ -402,7 +410,17 @@ EXPORT_SYMBOL(drm_modeset_lock_interruptible);
  */
 void drm_modeset_unlock(struct drm_modeset_lock *lock)
 {
-	list_del_init(&lock->head);
+	struct drm_modeset_lock_info *info;
+
+	/* undo in reverse order */
+	if (!list_empty(&lock->locked)) {
+		info = list_last_entry(&lock->locked,
+				struct drm_modeset_lock_info, lock_entry);
+		list_del_init(&info->lock_entry);
+		if (info->ctx)
+			list_del_init(&info->ctx_entry);
+		kfree(info);
+	}
 	ww_mutex_unlock(&lock->mutex);
 }
 EXPORT_SYMBOL(drm_modeset_unlock);
diff --git a/sys/dev/drm/include/drm/drm_modeset_lock.h b/sys/dev/drm/include/drm/drm_modeset_lock.h
index 1e88695784..d41f7dc73a 100644
--- a/sys/dev/drm/include/drm/drm_modeset_lock.h
+++ b/sys/dev/drm/include/drm/drm_modeset_lock.h
@@ -50,7 +50,7 @@ struct drm_modeset_acquire_ctx {
 	struct drm_modeset_lock *contended;
 
 	/**
-	 * list of held locks (drm_modeset_lock)
+	 * list of held locks (drm_modeset_lock_info)
 	 */
 	struct list_head locked;
 
@@ -60,6 +60,16 @@ struct drm_modeset_acquire_ctx {
 	bool trylock_only;
 };
 
+/*
+ * Keep track of extra locks.
+ */
+struct drm_modeset_lock_info {
+	struct list_head ctx_entry;
+	struct list_head lock_entry;
+	struct drm_modeset_lock *lock;
+	struct drm_modeset_acquire_ctx *ctx;
+};
+
 /**
  * struct drm_modeset_lock - used for locking modeset resources.
  * @mutex: resource locking
@@ -78,7 +88,7 @@ struct drm_modeset_lock {
 	 * Resources that are locked as part of an atomic update are added
 	 * to a list (so we know what to unlock at the end).
 	 */
-	struct list_head head;
+	struct list_head locked;
 };
 
 extern struct ww_class crtc_ww_class;
@@ -97,7 +107,7 @@ int drm_modeset_backoff_interruptible(struct drm_modeset_acquire_ctx *ctx);
 static inline void drm_modeset_lock_init(struct drm_modeset_lock *lock)
 {
 	ww_mutex_init(&lock->mutex, &crtc_ww_class);
-	INIT_LIST_HEAD(&lock->head);
+	INIT_LIST_HEAD(&lock->locked);
 }
 
 /**
@@ -106,7 +116,7 @@ static inline void drm_modeset_lock_init(struct drm_modeset_lock *lock)
  */
 static inline void drm_modeset_lock_fini(struct drm_modeset_lock *lock)
 {
-	WARN_ON(!list_empty(&lock->head));
+	WARN_ON(!list_empty(&lock->locked));
 }
 
 /**