[PATCH] drm/nouveau: Document weird looking bugfix

[Philipp Stanner <phasta@kernel.org>]

commit c8a5d5ea3ba6 ("nouveau: fix client work fence deletion race")
fixed a race. To do so, it replaced the automatically locking
dma_fence_is_signaled() with manual locks plus
dma_fence_is_signaled_locked().

For someone browsing through the code, this reads very much like a
cleanup or rework leftover. Future contributors and / or new maintainers
not familiar with the history might be tempted to remove that bugfix.

Document the bugfix.

Signed-off-by: Philipp Stanner <phasta@kernel.org>
---
(I did not test this)
---
 drivers/gpu/drm/nouveau/nouveau_drm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/nouveau/nouveau_drm.c b/drivers/gpu/drm/nouveau/nouveau_drm.c
index 42a81166f3a9..519a0c164a72 100644
--- a/drivers/gpu/drm/nouveau/nouveau_drm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_drm.c
@@ -159,6 +159,13 @@ nouveau_cli_work_ready(struct dma_fence *fence)
 	unsigned long flags;
 	bool ret = true;
 
+	/*
+	 * This is not a cleanup / rework leftover, but a bugfix to prevent a
+	 * race with someone signalling the fence. The locked
+	 * dma_fence_is_signaled() cannot be used. The dma_fence implementation
+	 * is not fully synchronized with locks, but also uses atomic bits,
+	 * which can cause the dma_fence_put() below to be executed too soon.
+	 */
 	dma_fence_lock_irqsave(fence, flags);
 	if (!dma_fence_is_signaled_locked(fence))
 		ret = false;
-- 
2.54.0