From 2facdd6002ad67357dd7f77a388ae602bc910ace Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Thomas=20Hellstr=C3=B6m?= <thomas.hellstrom@linux.intel.com>
Date: Fri, 28 Apr 2023 14:52:32 +0200
Subject: [PATCH] dma-buf/dma-fence: Use a successful read_trylock() annotation
 for dma_fence_begin_signalling()
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Condsider the following call sequence:

/* Upper layer */
dma_fence_begin_signalling();
lock(tainted_shared_lock);
/* Driver callback */
dma_fence_begin_signalling();
...

The driver might here use a utility that is annotated as intended for the
dma-fence signalling critical path. Now if the upper layer isn't correctly
annotated yet for whatever reason, resulting in

/* Upper layer */
lock(tainted_shared_lock);
/* Driver callback */
dma_fence_begin_signalling();

We will receive a false lockdep locking order violation notification from
dma_fence_begin_signalling(). However entering a dma-fence signalling
critical section itself doesn't block and could not cause a deadlock.

So use a successful read_trylock() annotation instead for
dma_fence_begin_signalling(). That will make sure that the locking order
is correctly registered in the first case, and doesn't register any
locking order in the second case.

The alternative is of course to make sure that the "Upper layer" is always
correctly annotated. But experience shows that's not easily achievable
in all cases.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230428125233.228353-1-thomas.hellstrom@linux.intel.com
---
 drivers/dma-buf/dma-fence.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 0393a9bba3a8a..f8303ae99acfd 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -309,8 +309,8 @@ bool dma_fence_begin_signalling(void)
 	if (in_atomic())
 		return true;
 
-	/* ... and non-recursive readlock */
-	lock_acquire(&dma_fence_lockdep_map, 0, 0, 1, 1, NULL, _RET_IP_);
+	/* ... and non-recursive successful read_trylock */
+	lock_acquire(&dma_fence_lockdep_map, 0, 1, 1, 1, NULL, _RET_IP_);
 
 	return false;
 }
@@ -341,7 +341,7 @@ void __dma_fence_might_wait(void)
 	lock_map_acquire(&dma_fence_lockdep_map);
 	lock_map_release(&dma_fence_lockdep_map);
 	if (tmp)
-		lock_acquire(&dma_fence_lockdep_map, 0, 0, 1, 1, NULL, _THIS_IP_);
+		lock_acquire(&dma_fence_lockdep_map, 0, 1, 1, 1, NULL, _THIS_IP_);
 }
 #endif
 
-- 
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