From 309a5482fa9eb7bc754bf95a2cd89091b01c33d2 Mon Sep 17 00:00:00 2001
From: Jason Ekstrand <jason@jlekstrand.net>
Date: Fri, 25 Aug 2017 10:52:20 -0700
Subject: [PATCH] drm/syncobj: Add a race-free drm_syncobj_fence_get helper (v2)
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Git-commit: 309a5482fa9eb7bc754bf95a2cd89091b01c33d2
Patch-mainline: v4.14-rc1
References: FATE#322643 bsc#1055900

The atomic exchange operation in drm_syncobj_replace_fence is sufficient
for the case where it races with itself.  However, if you have a race
between a replace_fence and dma_fence_get(syncobj->fence), you may end
up with the entire replace_fence happening between the point in time
where the one thread gets the syncobj->fence pointer and when it calls
dma_fence_get() on it.  If this happens, then the reference may be
dropped before we get a chance to get a new one.  The new helper uses
dma_fence_get_rcu_safe to get rid of the race.

This is also needed because it allows us to do a bit more than just get
a reference in drm_syncobj_fence_get should we wish to do so.

V2: - RCU isn't that scary - Call rcu_read_lock/unlock - Don't rename fence to _fence - Make the helper static inline

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Christian König <christian.koenig@amd.com> (v1)
Signed-off-by: Dave Airlie <airlied@redhat.com>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/gpu/drm/drm_syncobj.c |    2 +-
 include/drm/drm_syncobj.h     |   12 ++++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -105,7 +105,7 @@ int drm_syncobj_find_fence(struct drm_fi
 	if (!syncobj)
 		return -ENOENT;
 
-	*fence = dma_fence_get(syncobj->fence);
+	*fence = drm_syncobj_fence_get(syncobj);
 	if (!*fence) {
 		ret = -EINVAL;
 	}
--- a/include/drm/drm_syncobj.h
+++ b/include/drm/drm_syncobj.h
@@ -77,6 +77,18 @@ drm_syncobj_put(struct drm_syncobj *obj)
 	kref_put(&obj->refcount, drm_syncobj_free);
 }
 
+static inline struct dma_fence *
+drm_syncobj_fence_get(struct drm_syncobj *syncobj)
+{
+	struct dma_fence *fence;
+
+	rcu_read_lock();
+	fence = dma_fence_get_rcu_safe(&syncobj->fence);
+	rcu_read_unlock();
+
+	return fence;
+}
+
 struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private,
 				     u32 handle);
 void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,