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Tue, 15 Sep 2026 08:33:39 -0700 From: Edward Srouji Date: Tue, 15 Sep 2026 18:33:29 +0300 Subject: [PATCH rdma-next 2/4] RDMA/mlx4: Use unsigned comparison in the CQ cleanup loop Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-ID: <20260915-fix-cq-cleanup-v1-2-e991944cf898@nvidia.com> References: <20260915-fix-cq-cleanup-v1-0-e991944cf898@nvidia.com> In-Reply-To: <20260915-fix-cq-cleanup-v1-0-e991944cf898@nvidia.com> To: Leon Romanovsky , Jason Gunthorpe , "Nathan Chancellor" , Nick Desaulniers , Bill Wendling , Justin Stitt , Yishai Hadas , Chengchang Tang , Junxian Huang CC: , , , Edward Srouji X-Mailer: b4 0.14.2 X-Developer-Signature: v=1; a=ed25519-sha256; t=1789486411; l=2285; i=edwards@nvidia.com; s=20251029; h=from:subject:message-id; bh=qDCVVIuWwQEgw505znA0f/vkbbnQjVwmxxKgONCC/ck=; b=Qva+HWFfba7/XrcvEJEwOt13O4AEaP4x3pkx5Y8GlzpeB/9gklVHJbrKCmgJdXgx5LMn4qs47 mFl+cpJJA6dBH0eWk0JhbkaVGApVc5vZa+tw/UwGGXQRKTYKtA3ezwL X-Developer-Key: i=edwards@nvidia.com; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: NHfpuHuaut20jgjVztmAmE+u9VwQEn6xrNxaYa957W4vYAggaUEcam/8KtJwkrzh4Xq+GcNgxs4YOp4tkqgIdc0Kt8nJPvkt3NxklMKopchS2nZGEfh+0ciCzKzf36TEugM1y1uoo9Za96s0yFlfPK51ZaxFTZD5HmPd5iY4WpvSBNUpJiRuSvRSVzRhM+gfbsZgPFEDHBkwVNFNbXTf9K3oD9Ei8TBRhNS1xABwieZDygEUS/WL721TXCUVYAtEmHkbWa/7AXNqMA5lDf/K80qSDCnH38mW7BnYT1RBNUTg+mzZjm52iBmwIUyeDB6T5+jVn17gcc5gToqsWozH5iqdH0DyohYYExPcGKYYKmFr8Zrlx0mmnZ7U45waPcci6p4ycpg63Oi0yxO+YflqByK1OL87Z3r5BfeVfCx/duVStw1Po3ufH+FRkTljHTvO X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 15 Sep 2026 15:34:16.7121 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 54d9eff0-074f-46ca-ad3b-08df133ecda6 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.160];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: BL02EPF0002992D.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN7PPFFC4F04B28 From: Yishai Hadas __mlx4_ib_cq_clean() sweeps the CQ backwards from the producer index down to the consumer index: while ((int) --prod_index - (int) cq->mcq.cons_index >= 0) Both indexes are free running u32 counters, so the comparison has to be done modulo 2^32. Casting each operand to int and subtracting does not do that: the subtraction overflows whenever the two indexes straddle 2^31, which is undefined behaviour, and a compiler that assumes signed overflow cannot occur is free to discard the subtraction and fold the expression into a plain signed comparison. That comparison is not wraparound safe. The kernel is built with -fno-strict-overflow, so gcc and clang both retain the subtraction today and the generated code is unaffected; there is no known user-visible impact from the current code. Still, correctness here shouldn't depend on that build flag. Replace the loop with a plain unsigned equality check instead: while (prod_index != cq->mcq.cons_index) { --prod_index; ... The preceding forward scan starts prod_index at cons_index and stops no later than cons_index + cq->ibcq.cqe, so the two indexes are at most cq->ibcq.cqe apart. Decrementing prod_index reaches cons_index in exactly that many iterations regardless of whether either counter has wrapped, because the loop no longer compares magnitudes at all. Signed-off-by: Yishai Hadas Signed-off-by: Edward Srouji --- drivers/infiniband/hw/mlx4/cq.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/drivers/infiniband/hw/mlx4/cq.c b/drivers/infiniband/hw/mlx4/cq.c index 8879124697420c2ba03622c2af3b350808dc0aa2..98e3eb5cf96e54efdec6e4d8c588fc865742337a 100644 --- a/drivers/infiniband/hw/mlx4/cq.c +++ b/drivers/infiniband/hw/mlx4/cq.c @@ -989,7 +989,8 @@ void __mlx4_ib_cq_clean(struct mlx4_ib_cq *cq, u32 qpn, struct mlx4_ib_srq *srq) * Now sweep backwards through the CQ, removing CQ entries * that match our QP by copying older entries on top of them. */ - while ((int) --prod_index - (int) cq->mcq.cons_index >= 0) { + while (prod_index != cq->mcq.cons_index) { + --prod_index; cqe = get_cqe(cq, prod_index & cq->ibcq.cqe); cqe += cqe_inc; -- 2.49.0