[PATCH] nouveau: fix client work fence deletion race

[Dave Airlie <airlied@gmail.com>]

From: Dave Airlie <airlied@redhat.com>

This seems to have existed for ever but is now more apparant after
9bff18d13473a9fdf81d5158248472a9d8ecf2bd (drm/ttm: use per BO cleanup workers)

My analysis:
two threads are running,
one in the irq signalling the fence, in dma_fence_signal_timestamp_locked,
it has done the DMA_FENCE_FLAG_SIGNALLED_BIT setting, but hasn't yet reached the callbacks.

second thread in nouveau_cli_work_ready, where it sees the fence is signalled, so then puts the
fence, cleanups the object and frees the work item, which contains the callback.

thread one goes again and tries to call the callback and causes the use-after-free.

Proposed fix:
lock the fence signalled check in nouveau_cli_work_ready, so either the callbacks are done
or the memory is freed.

Signed-off-by: Dave Airlie <airlied@redhat.com>
---
 drivers/gpu/drm/nouveau/nouveau_drm.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_drm.c b/drivers/gpu/drm/nouveau/nouveau_drm.c
index cc7c5b4a05fd..1a45be769848 100644
--- a/drivers/gpu/drm/nouveau/nouveau_drm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_drm.c
@@ -137,10 +137,16 @@ nouveau_name(struct drm_device *dev)
 static inline bool
 nouveau_cli_work_ready(struct dma_fence *fence)
 {
-	if (!dma_fence_is_signaled(fence))
-		return false;
-	dma_fence_put(fence);
-	return true;
+	unsigned long flags;
+	bool ret = true;
+	spin_lock_irqsave(fence->lock, flags);
+	if (!dma_fence_is_signaled_locked(fence))
+		ret = false;
+	spin_unlock_irqrestore(fence->lock, flags);
+
+	if (ret == true)
+		dma_fence_put(fence);
+	return ret;
 }
 
 static void
-- 
2.40.1