linux-kernel - [PATCH] dma-buf: insert memory barrier before updating num

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Message-ID: <20250512111231.GA1265534@au1-maretx-p37.eng.sarc.samsung.com>
Date: Mon, 12 May 2025 06:12:31 -0500
From: Hyejeong Choi <hjeong.choi@...sung.com>
To: sumit.semwal@...aro.org, christian.koenig@....com
Cc: linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	hjeong.choi@...sung.com
Subject: [PATCH] dma-buf: insert memory barrier before updating num_fences

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@...sung.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 5f8d010516f0..52af5c7430da 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -320,8 +320,9 @@ void dma_resv_add_fence(struct dma_resv *obj, struct dma_fence *fence,
 	count++;
 
 	dma_resv_list_set(fobj, i, fence, usage);
-	/* pointer update must be visible before we extend the num_fences */
-	smp_store_mb(fobj->num_fences, count);
+	/* fence update must be visible before we extend the num_fences */
+	smp_wmb();
+	WRITE_ONCE(fobj->num_fences, count);
 }
 EXPORT_SYMBOL(dma_resv_add_fence);
 
-- 
2.31.1