[PATCH 1/4] sync_file: add a mutex to protect fence and callback members. (v22)

[Dave Airlie <airlied@gmail.com>]

From: Dave Airlie <airlied@redhat.com>

This isn't needed currently, but to reuse sync file for Vulkan
permanent shared semaphore semantics, we need to be able to swap
the fence backing a sync file. This patch adds a mutex to the
sync file and uses to protect accesses to the fence and cb members.

v1.1: fix the locking (Julia Lawall).
v2: use rcu try one

Signed-off-by: Dave Airlie <airlied@redhat.com>
---
 drivers/dma-buf/sync_file.c | 82 +++++++++++++++++++++++++++++++++++----------
 include/linux/sync_file.h   |  5 ++-
 2 files changed, 69 insertions(+), 18 deletions(-)

diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
index 2321035..8b34f21 100644
--- a/drivers/dma-buf/sync_file.c
+++ b/drivers/dma-buf/sync_file.c
@@ -28,6 +28,10 @@
 
 static const struct file_operations sync_file_fops;
 
+#define sync_file_held(obj) lockdep_is_held(&(obj)->lock)
+#define sync_file_assert_held(obj) \
+	lockdep_assert_held(&(obj)->lock)
+
 static struct sync_file *sync_file_alloc(void)
 {
 	struct sync_file *sync_file;
@@ -47,6 +51,9 @@ static struct sync_file *sync_file_alloc(void)
 
 	INIT_LIST_HEAD(&sync_file->cb.node);
 
+	RCU_INIT_POINTER(sync_file->fence, NULL);
+
+	mutex_init(&sync_file->lock);
 	return sync_file;
 
 err:
@@ -80,7 +87,9 @@ struct sync_file *sync_file_create(struct dma_fence *fence)
 	if (!sync_file)
 		return NULL;
 
-	sync_file->fence = dma_fence_get(fence);
+	dma_fence_get(fence);
+
+	RCU_INIT_POINTER(sync_file->fence, fence);
 
 	snprintf(sync_file->name, sizeof(sync_file->name), "%s-%s%llu-%d",
 		 fence->ops->get_driver_name(fence),
@@ -124,13 +133,26 @@ struct dma_fence *sync_file_get_fence(int fd)
 	if (!sync_file)
 		return NULL;
 
-	fence = dma_fence_get(sync_file->fence);
+	if (!rcu_access_pointer(sync_file->fence))
+		return NULL;
+
+	rcu_read_lock();
+	fence = dma_fence_get_rcu_safe(&sync_file->fence);
+	rcu_read_unlock();
+
 	fput(sync_file->file);
 
 	return fence;
 }
 EXPORT_SYMBOL(sync_file_get_fence);
 
+static inline struct dma_fence *
+sync_file_get_fence_locked(struct sync_file *sync_file)
+{
+	return rcu_dereference_protected(sync_file->fence,
+					 sync_file_held(sync_file));
+}
+
 static int sync_file_set_fence(struct sync_file *sync_file,
 			       struct dma_fence **fences, int num_fences)
 {
@@ -143,7 +165,7 @@ static int sync_file_set_fence(struct sync_file *sync_file,
 	 * we own the reference of the dma_fence_array creation.
 	 */
 	if (num_fences == 1) {
-		sync_file->fence = fences[0];
+		RCU_INIT_POINTER(sync_file->fence, fences[0]);
 		kfree(fences);
 	} else {
 		array = dma_fence_array_create(num_fences, fences,
@@ -152,17 +174,20 @@ static int sync_file_set_fence(struct sync_file *sync_file,
 		if (!array)
 			return -ENOMEM;
 
-		sync_file->fence = &array->base;
+		RCU_INIT_POINTER(sync_file->fence, &array->base);
 	}
 
 	return 0;
 }
 
+/* must be called with sync_file lock taken */
 static struct dma_fence **get_fences(struct sync_file *sync_file,
 				     int *num_fences)
 {
-	if (dma_fence_is_array(sync_file->fence)) {
-		struct dma_fence_array *array = to_dma_fence_array(sync_file->fence);
+	struct dma_fence *fence = sync_file_get_fence_locked(sync_file);
+
+	if (dma_fence_is_array(fence)) {
+		struct dma_fence_array *array = to_dma_fence_array(fence);
 
 		*num_fences = array->num_fences;
 		return array->fences;
@@ -204,10 +229,13 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
 	if (!sync_file)
 		return NULL;
 
+	mutex_lock(&a->lock);
+	mutex_lock(&b->lock);
 	a_fences = get_fences(a, &a_num_fences);
 	b_fences = get_fences(b, &b_num_fences);
-	if (a_num_fences > INT_MAX - b_num_fences)
-		return NULL;
+	if (a_num_fences > INT_MAX - b_num_fences) {
+		goto unlock;
+	}
 
 	num_fences = a_num_fences + b_num_fences;
 
@@ -268,11 +296,17 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
 		goto err;
 	}
 
+	mutex_unlock(&b->lock);
+	mutex_unlock(&a->lock);
+
 	strlcpy(sync_file->name, name, sizeof(sync_file->name));
 	return sync_file;
 
 err:
 	fput(sync_file->file);
+unlock:
+	mutex_unlock(&b->lock);
+	mutex_unlock(&a->lock);
 	return NULL;
 
 }
@@ -281,10 +315,15 @@ static void sync_file_free(struct kref *kref)
 {
 	struct sync_file *sync_file = container_of(kref, struct sync_file,
 						     kref);
+	struct dma_fence *fence;
+
+	fence = rcu_dereference_protected(sync_file->fence, 1);
+	if (fence) {
+		if (test_bit(POLL_ENABLED, &fence->flags))
+			dma_fence_remove_callback(fence, &sync_file->cb);
+		dma_fence_put(fence);
+	}
 
-	if (test_bit(POLL_ENABLED, &sync_file->fence->flags))
-		dma_fence_remove_callback(sync_file->fence, &sync_file->cb);
-	dma_fence_put(sync_file->fence);
 	kfree(sync_file);
 }
 
@@ -299,16 +338,20 @@ static int sync_file_release(struct inode *inode, struct file *file)
 static unsigned int sync_file_poll(struct file *file, poll_table *wait)
 {
 	struct sync_file *sync_file = file->private_data;
+	unsigned int ret_val;
 
 	poll_wait(file, &sync_file->wq, wait);
 
+	mutex_lock(&sync_file->lock);
 	if (!test_and_set_bit(POLL_ENABLED, &sync_file->fence->flags)) {
 		if (dma_fence_add_callback(sync_file->fence, &sync_file->cb,
 					   fence_check_cb_func) < 0)
 			wake_up_all(&sync_file->wq);
 	}
+	ret_val = dma_fence_is_signaled(sync_file->fence) ? POLLIN : 0;
+	mutex_unlock(&sync_file->lock);
 
-	return dma_fence_is_signaled(sync_file->fence) ? POLLIN : 0;
+	return ret_val;
 }
 
 static long sync_file_ioctl_merge(struct sync_file *sync_file,
@@ -393,6 +436,7 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
 	if (info.flags || info.pad)
 		return -EINVAL;
 
+	mutex_lock(&sync_file->lock);
 	fences = get_fences(sync_file, &num_fences);
 
 	/*
@@ -404,13 +448,17 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
 	if (!info.num_fences)
 		goto no_fences;
 
-	if (info.num_fences < num_fences)
-		return -EINVAL;
+	if (info.num_fences < num_fences) {
+		ret = -EINVAL;
+		goto out;
+	}
 
 	size = num_fences * sizeof(*fence_info);
 	fence_info = kzalloc(size, GFP_KERNEL);
-	if (!fence_info)
-		return -ENOMEM;
+	if (!fence_info) {
+		ret = -ENOMEM;
+		goto out;
+	}
 
 	for (i = 0; i < num_fences; i++)
 		sync_fill_fence_info(fences[i], &fence_info[i]);
@@ -433,7 +481,7 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
 
 out:
 	kfree(fence_info);
-
+	mutex_unlock(&sync_file->lock);
 	return ret;
 }
 
diff --git a/include/linux/sync_file.h b/include/linux/sync_file.h
index 3e3ab84..006412f 100644
--- a/include/linux/sync_file.h
+++ b/include/linux/sync_file.h
@@ -30,6 +30,7 @@
  * @wq:			wait queue for fence signaling
  * @fence:		fence with the fences in the sync_file
  * @cb:			fence callback information
+ * @lock:               mutex to protect fence/cb - used for semaphores
  */
 struct sync_file {
 	struct file		*file;
@@ -41,8 +42,10 @@ struct sync_file {
 
 	wait_queue_head_t	wq;
 
-	struct dma_fence	*fence;
+	struct dma_fence __rcu	*fence;
 	struct dma_fence_cb cb;
+	/* protects the fence pointer and cb */
+	struct mutex lock;
 };
 
 #define POLL_ENABLED DMA_FENCE_FLAG_USER_BITS
-- 
2.7.4

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