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[103.229.16.4]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2a0993ab61dsm118846515ad.46.2025.12.16.22.51.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 16 Dec 2025 22:51:32 -0800 (PST) From: Zhongqiu Han To: andersson@kernel.org, mathieu.poirier@linaro.org, corbet@lwn.net, rusty@rustcorp.com.au, ohad@wizery.com Cc: linux-remoteproc@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, zhongqiu.han@oss.qualcomm.com Subject: [RFC PATCH 5/5] rpmsg: virtio: Optimize endpoint lookup in RX path with RCU Date: Wed, 17 Dec 2025 14:51:12 +0800 Message-ID: <20251217065112.18392-6-zhongqiu.han@oss.qualcomm.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251217065112.18392-1-zhongqiu.han@oss.qualcomm.com> References: <20251217065112.18392-1-zhongqiu.han@oss.qualcomm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Proofpoint-GUID: 0bn7zb1gqNyP3GkkLBaQFwFwQsImuymq X-Proofpoint-ORIG-GUID: 0bn7zb1gqNyP3GkkLBaQFwFwQsImuymq X-Authority-Analysis: v=2.4 cv=f/RFxeyM c=1 sm=1 tr=0 ts=694252f6 cx=c_pps a=JL+w9abYAAE89/QcEU+0QA==:117 a=nuhDOHQX5FNHPW3J6Bj6AA==:17 a=wP3pNCr1ah4A:10 a=s4-Qcg_JpJYA:10 a=VkNPw1HP01LnGYTKEx00:22 a=EUspDBNiAAAA:8 a=4XvfFkW6Ni2-fQWSjOsA:9 a=324X-CrmTo6CU4MGRt3R:22 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMjE3MDA1MyBTYWx0ZWRfXzgm/wJp0lGsf BrYRiuEhMruUOQm5GYccyZImzAmArFOcKO7ETRANSEV2CQ7mOuw0kyVc83BqhfrpCXW4DaKfC5i CjNlj1vt67i9PFSgumWfA6IDDi099wbjo0Ntb2YDK+Ke1Cu1onP0x1Bw8r7IwEkGsR4Vh1K+rD6 wchwgRin08LJg4mtMo5rILEpHhqVgNAKJpSUUaziG7ZiDTt0UEF2la2HCCFxdYpWDdV8IMysH65 uwhW+95CcvaU+weRaKH/wMKKZnnRORkslQQbqL4WOrchRYHB2kg4eNJzxxBPY15VZ8UKrmOG1Ix NDQv3eRdsr21GjqoWT2IM8FfRydHC3vSjTNUHCHWib7kDfCWHE1jRJetGtoES9EvaxFOR5wT1UM gJuBlwuI27L53G36sQ+c8XBNcGsquQ== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.100.49 definitions=2025-12-17_01,2025-12-16_05,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 phishscore=0 suspectscore=0 lowpriorityscore=0 clxscore=1015 priorityscore=1501 malwarescore=0 adultscore=0 bulkscore=0 impostorscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2510240001 definitions=main-2512170053 Content-Type: text/plain; charset="utf-8" Endpoint lookup in the receive path acts as a demultiplexer routing incoming messages to the appropriate endpoint. This is a read-heavy operation (frequent message receives) with infrequent writes (endpoint creation/destruction). Since idr_find() is safe under RCU read-side protection, RCU can be used to optimize this path. Convert endpoint lookup to use RCU: - Read path: Use rcu_read_lock/unlock for lockless lookup - Destroy path: Add synchronize_rcu() after endpoint removal This reduces lock contention in the hot receive path. RCU safety note: When idr_alloc() returns, the endpoint becomes immediately visible to idr_find(), but ept->addr might not yet be set. This creates a theoretical window where RX could find an endpoint with uninitialized addr. This is safe because: 1) When endpoints are created via rpmsg core callbacks, initialization completes before announce_create() is sent. Remote processors only send messages after receiving the announcement. 2) For manually created endpoints, drivers control timing and typically do not announce until ready. Thus, messages only arrive after ept->addr is initialized, making this RCU optimization safe. No functional change except reduced contention. Signed-off-by: Zhongqiu Han --- drivers/rpmsg/virtio_rpmsg_bus.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_= bus.c index 79d983055b4d..4cbb8a8aaec5 100644 --- a/drivers/rpmsg/virtio_rpmsg_bus.c +++ b/drivers/rpmsg/virtio_rpmsg_bus.c @@ -17,6 +17,7 @@ #include #include #include +#include #include #include #include @@ -297,6 +298,12 @@ __rpmsg_destroy_ept(struct virtproc_info *vrp, struct = rpmsg_endpoint *ept) idr_remove(&vrp->endpoints, ept->addr); mutex_unlock(&vrp->endpoints_lock); =20 + /* + * Wait for any ongoing RCU read-side critical sections to complete. + * This ensures no one is accessing the endpoint after removal. + */ + synchronize_rcu(); + /* make sure in-flight inbound messages won't invoke cb anymore */ mutex_lock(&ept->cb_lock); ept->cb =3D NULL; @@ -680,7 +687,7 @@ static int rpmsg_recv_single(struct virtproc_info *vrp,= struct device *dev, } =20 /* use the dst addr to fetch the callback of the appropriate user */ - mutex_lock(&vrp->endpoints_lock); + rcu_read_lock(); =20 ept =3D idr_find(&vrp->endpoints, __rpmsg32_to_cpu(little_endian, msg->ds= t)); =20 @@ -688,7 +695,7 @@ static int rpmsg_recv_single(struct virtproc_info *vrp,= struct device *dev, if (ept) kref_get(&ept->refcount); =20 - mutex_unlock(&vrp->endpoints_lock); + rcu_read_unlock(); =20 if (ept) { /* make sure ept->cb doesn't go away while we use it */ --=20 2.43.0