[PATCH v4] dma-buf/sync-file: Defer creation of sync_file->name

[Chris Wilson <chris@chris-wilson.co.uk>]

Constructing the name takes the majority of the time for allocating a
sync_file to wrap a fence, and the name is very rarely used (only via
the sync_file status user interface). To reduce the impact on the common
path (that of creating sync_file to pass around), defer the construction
of the name until it is first used.

v2: Update kerneldoc (kbuild test robot)
v3: sync_debug.c was peeking at the name
v4: Comment upon the potential race between two users of
sync_file_get_name() and claim that such a race is below the level of
notice. However, to prevent any future nuisance, use a global spinlock
to serialize the assignment of the name.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/dma-buf/sync_debug.c |  3 ++-
 drivers/dma-buf/sync_file.c  | 54 ++++++++++++++++++++++++++++++++++++++------
 include/linux/sync_file.h    |  5 ++--
 3 files changed, 52 insertions(+), 10 deletions(-)

diff --git a/drivers/dma-buf/sync_debug.c b/drivers/dma-buf/sync_debug.c
index 4b1731ee7458..9a93f1085c63 100644
--- a/drivers/dma-buf/sync_debug.c
+++ b/drivers/dma-buf/sync_debug.c
@@ -134,7 +134,8 @@ static void sync_print_sync_file(struct seq_file *s,
 {
 	int i;
 
-	seq_printf(s, "[%p] %s: %s\n", sync_file, sync_file->name,
+	seq_printf(s, "[%p] %s: %s\n", sync_file,
+		   sync_file_get_name(sync_file),
 		   sync_status_str(dma_fence_get_status(sync_file->fence)));
 
 	if (dma_fence_is_array(sync_file->fence)) {
diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
index c9eb4997cfcc..0d54de79ba76 100644
--- a/drivers/dma-buf/sync_file.c
+++ b/drivers/dma-buf/sync_file.c
@@ -80,11 +80,6 @@ struct sync_file *sync_file_create(struct dma_fence *fence)
 
 	sync_file->fence = dma_fence_get(fence);
 
-	snprintf(sync_file->name, sizeof(sync_file->name), "%s-%s%llu-%d",
-		 fence->ops->get_driver_name(fence),
-		 fence->ops->get_timeline_name(fence), fence->context,
-		 fence->seqno);
-
 	return sync_file;
 }
 EXPORT_SYMBOL(sync_file_create);
@@ -129,6 +124,51 @@ struct dma_fence *sync_file_get_fence(int fd)
 }
 EXPORT_SYMBOL(sync_file_get_fence);
 
+/**
+ * sync_file_get_name - get the name of the sync_file
+ * @sync_file:		sync_file to get the fence from
+ *
+ * Each sync_file may have a name assigned either by the user (when merging
+ * sync_files together) or created from the fence it contains. However,
+ * construction of the name is deferred until first use.
+ *
+ * Returns: a string representing the name
+ */
+char *sync_file_get_name(struct sync_file *sync_file)
+{
+	static DEFINE_SPINLOCK(name_lock);
+
+	if (!sync_file->user_name[0]) {
+		char buf[sizeof(sync_file->user_name)];
+		struct dma_fence *fence;
+		int len;
+
+		rcu_read_lock();
+		fence = sync_file->fence;
+		len = scnprintf(buf, sizeof(buf), "%s-%s%llu-%d",
+				fence->ops->get_driver_name(sync_file->fence),
+				fence->ops->get_timeline_name(sync_file->fence),
+				fence->context,
+				fence->seqno);
+
+		/* This is mildly racy, so we stage the printf into a local
+		 * buffer and do the copy with preempt disabled to trim the
+		 * race down to a few cycles. To completely eliminate the
+		 * race, we do the final assignment underneath a global
+		 * spinlock. A global lock should be adequate here for this
+		 * rare path (called from the middle of a user sw_sync status
+		 * ioctl and from debugfs).
+		 */
+		spin_lock(&name_lock);
+		if (!sync_file->user_name[0])
+			memcpy(sync_file->user_name, buf, len + 1);
+		spin_unlock(&name_lock);
+		rcu_read_unlock();
+	}
+
+	return sync_file->user_name;
+}
+
 static int sync_file_set_fence(struct sync_file *sync_file,
 			       struct dma_fence **fences, int num_fences)
 {
@@ -266,7 +306,7 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
 		goto err;
 	}
 
-	strlcpy(sync_file->name, name, sizeof(sync_file->name));
+	strlcpy(sync_file->user_name, name, sizeof(sync_file->user_name));
 	return sync_file;
 
 err:
@@ -419,7 +459,7 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
 	}
 
 no_fences:
-	strlcpy(info.name, sync_file->name, sizeof(info.name));
+	strlcpy(info.name, sync_file_get_name(sync_file), sizeof(info.name));
 	info.status = dma_fence_is_signaled(sync_file->fence);
 	info.num_fences = num_fences;
 
diff --git a/include/linux/sync_file.h b/include/linux/sync_file.h
index d37beefdfbd5..0f3f05325c84 100644
--- a/include/linux/sync_file.h
+++ b/include/linux/sync_file.h
@@ -23,7 +23,7 @@
 /**
  * struct sync_file - sync file to export to the userspace
  * @file:		file representing this fence
- * @name:		name of sync_file.  Useful for debugging
+ * @user_name:		name of sync_file.  Useful for debugging
  * @sync_file_list:	membership in global file list
  * @wq:			wait queue for fence signaling
  * @fence:		fence with the fences in the sync_file
@@ -31,7 +31,7 @@
  */
 struct sync_file {
 	struct file		*file;
-	char			name[32];
+	char			user_name[32];
 #ifdef CONFIG_DEBUG_FS
 	struct list_head	sync_file_list;
 #endif
@@ -46,5 +46,6 @@ struct sync_file {
 
 struct sync_file *sync_file_create(struct dma_fence *fence);
 struct dma_fence *sync_file_get_fence(int fd);
+char *sync_file_get_name(struct sync_file *sync_file);
 
 #endif /* _LINUX_SYNC_H */
-- 
2.11.0

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