[PATCH 5.10] dma-buf: insert memory barrier before updating num_fences

[Jay Wang <wanjay@amazon.com>]

From: Hyejeong Choi <hjeong.choi@samsung.com>

commit 72c7d62583ebce7baeb61acce6057c361f73be4a upstream.

smp_store_mb() inserts memory barrier after storing operation.
It is different with what the comment is originally aiming so Null
pointer dereference can be happened if memory update is reordered.

Signed-off-by: Hyejeong Choi <hjeong.choi@samsung.com>
Fixes: a590d0fdbaa5 ("dma-buf: Update reservation shared_count after adding the new fence")
CC: stable@vger.kernel.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20250513020638.GA2329653@au1-maretx-p37.eng.sarc.samsung.com
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[Conflict resolved by applying changes from dma_resv_add_fence() in the original fix to dma_resv_add_shared_fence() in current code base]
Signed-off-by: Jay Wang <wanjay@amazon.com>
---
 drivers/dma-buf/dma-resv.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 789f72db097f..16eb0b389e08 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -291,8 +291,9 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
 
 replace:
 	RCU_INIT_POINTER(fobj->shared[i], fence);
-	/* pointer update must be visible before we extend the shared_count */
-	smp_store_mb(fobj->shared_count, count);
+	/* fence update must be visible before we extend the shared_count */
+	smp_wmb();
+	fobj->shared_count = count;
 
 	write_seqcount_end(&obj->seq);
 	dma_fence_put(old);
-- 
2.47.3