linux-kernel - [PATCH 5.8 199/464] dma-buf: fix dma-fence-chain out of order test

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Date:   Mon, 17 Aug 2020 17:12:32 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Lionel Landwerlin <lionel.g.landwerlin@...el.com>,
        Christian König <christian.koenig@....com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.8 199/464] dma-buf: fix dma-fence-chain out of order test

From: Lionel Landwerlin <lionel.g.landwerlin@...el.com>

[ Upstream commit 4cca2e641641767593583749dec26bfebd8f6f2d ]

There was probably a misunderstand on how the dma-fence-chain is
supposed to work or what dma_fence_chain_find_seqno() is supposed to
return.

dma_fence_chain_find_seqno() is here to give us the fence to wait upon
for a particular point in the timeline. The timeline progresses only
when all the points prior to a given number have completed.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@...el.com>
Fixes: dc2f7e67a28a5c ("dma-buf: Exercise dma-fence-chain under selftests")
Link: https://patchwork.freedesktop.org/patch/372960/
Signed-off-by: Christian König <christian.koenig@....com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/dma-buf/st-dma-fence-chain.c | 43 ++++++++++++++--------------
 1 file changed, 21 insertions(+), 22 deletions(-)

diff --git a/drivers/dma-buf/st-dma-fence-chain.c b/drivers/dma-buf/st-dma-fence-chain.c
index 5d45ba7ba3cd3..9525f7f561194 100644
--- a/drivers/dma-buf/st-dma-fence-chain.c
+++ b/drivers/dma-buf/st-dma-fence-chain.c
@@ -318,15 +318,16 @@ static int find_out_of_order(void *arg)
 		goto err;
 	}
 
-	if (fence && fence != fc.chains[1]) {
+	/*
+	 * We signaled the middle fence (2) of the 1-2-3 chain. The behavior
+	 * of the dma-fence-chain is to make us wait for all the fences up to
+	 * the point we want. Since fence 1 is still not signaled, this what
+	 * we should get as fence to wait upon (fence 2 being garbage
+	 * collected during the traversal of the chain).
+	 */
+	if (fence != fc.chains[0]) {
 		pr_err("Incorrect chain-fence.seqno:%lld reported for completed seqno:2\n",
-		       fence->seqno);
-
-		dma_fence_get(fence);
-		err = dma_fence_chain_find_seqno(&fence, 2);
-		dma_fence_put(fence);
-		if (err)
-			pr_err("Reported %d for finding self!\n", err);
+		       fence ? fence->seqno : 0);
 
 		err = -EINVAL;
 	}
@@ -415,20 +416,18 @@ static int __find_race(void *arg)
 		if (!fence)
 			goto signal;
 
-		err = dma_fence_chain_find_seqno(&fence, seqno);
-		if (err) {
-			pr_err("Reported an invalid fence for find-self:%d\n",
-			       seqno);
-			dma_fence_put(fence);
-			break;
-		}
-
-		if (fence->seqno < seqno) {
-			pr_err("Reported an earlier fence.seqno:%lld for seqno:%d\n",
-			       fence->seqno, seqno);
-			err = -EINVAL;
-			dma_fence_put(fence);
-			break;
+		/*
+		 * We can only find ourselves if we are on fence we were
+		 * looking for.
+		 */
+		if (fence->seqno == seqno) {
+			err = dma_fence_chain_find_seqno(&fence, seqno);
+			if (err) {
+				pr_err("Reported an invalid fence for find-self:%d\n",
+				       seqno);
+				dma_fence_put(fence);
+				break;
+			}
 		}
 
 		dma_fence_put(fence);
-- 
2.25.1