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Tue, 15 Sep 2026 08:33:43 -0700 From: Edward Srouji Date: Tue, 15 Sep 2026 18:33:30 +0300 Subject: [PATCH rdma-next 3/4] RDMA/mthca: 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-v1-3-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=2346; i=edwards@nvidia.com; s=20251029; h=from:subject:message-id; bh=m7xNUR84G+12GRtitfEsep8FMplbglDBsqZs9oE+rPo=; b=yA/QztXBc2fDbXFdqpOMr0+arQwYYcHEKM51DG+V5baLuqU0KW9TqKQZtrpNd2H2c8rxwiUUI bRlJUaLEjmqBjSDmsOFge4ClXy2oBVTTTbODj4bK6/MYdbzLGKe5lj5 X-Developer-Key: i=edwards@nvidia.com; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: rC0EgjpdCPuH1zU51O6oHUZ4T6tm6pA54KrYE8vEScyo0f/iAR8Z7zd2lOCNFaDtedAz+n9AtUMCFIAZWT0Tuwtfw3Iky26wwJKYSf0Av+a71KSNa1vUjnFfrD9Pyofe0a6pXn6NK8msf+nIl8I2OPT0z8rJZOXoOGRO3RXizBCD4zlhdxVbc6pPEGz/MtuhS1DU6CrkpEDwdGHKlvmfkZJjjfEErSbekdeFWZfGNVVf3SlMD5o7OX3ahpmUzWrpuwS6ny6Ed3K7WaNaC1Zw0ZZPWxz+KyBWT7ukF0MoaAouHnJxzTKhOmWzcy0zCNxX/B3lExI3Pg7c7nfkhLGgsIDmCwL7zbl9Ep3KH5rbez7Uo2CntqCUd44qEoAehPH48skJxYLhtvN5YvqqdQAxfSP26oCOLgVmaPZcU4DuVxgr1hbJ13zIV+badx/yE0pv X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 15 Sep 2026 15:34:19.2601 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 3c1699a4-4e01-422c-8e03-08df133ecf29 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: BL02EPF0002992C.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW4PR12MB5603 From: Yishai Hadas mthca_cq_clean() sweeps the CQ backwards from the producer index down to the consumer index: while ((int) --prod_index - (int) cq->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->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/mthca/mthca_cq.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/drivers/infiniband/hw/mthca/mthca_cq.c b/drivers/infiniband/hw/mthca/mthca_cq.c index 26c3408dcacaea9a0187257cc16de1b90f43fa40..39d4ff15849e7f8c2c3d47b4504306a4a4431c5d 100644 --- a/drivers/infiniband/hw/mthca/mthca_cq.c +++ b/drivers/infiniband/hw/mthca/mthca_cq.c @@ -300,7 +300,8 @@ void mthca_cq_clean(struct mthca_dev *dev, struct mthca_cq *cq, u32 qpn, * 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->cons_index >= 0) { + while (prod_index != cq->cons_index) { + --prod_index; cqe = get_cqe(cq, prod_index & cq->ibcq.cqe); if (cqe->my_qpn == cpu_to_be32(qpn)) { if (srq && is_recv_cqe(cqe)) -- 2.49.0