From b541dd554bc0442f7ff8c6cab6c5460c044913c8 Mon Sep 17 00:00:00 2001 From: Chuck Lever Date: Tue, 21 Nov 2023 11:40:13 -0500 Subject: svcrdma: Eliminate allocation of recv_ctxt objects in backchannel The svc_rdma_recv_ctxt free list uses a lockless list to avoid the need for a spin lock in the fast path. llist_del_first(), which is used by svc_rdma_recv_ctxt_get(), requires serialization, however, when there are multiple list producers that are unserialized. I mistakenly thought there was only one caller of svc_rdma_recv_ctxt_get() (svc_rdma_refresh_recvs()), thus explicit serialization would not be necessary. But there is another caller: svc_rdma_bc_sendto(), and these two are not serialized against each other. I haven't seen ill effects that I could directly ascribe to a lack of serialization. It's just an observation based on code audit. When DMA-mapping before sending a Reply, the passed-in struct svc_rdma_recv_ctxt is used only for its write and reply PCLs. These are currently always empty in the backchannel case. So, instead of passing a full svc_rdma_recv_ctxt object to svc_rdma_map_reply_msg(), let's pass in just the Write and Reply PCLs. This change makes it unnecessary for the backchannel to acquire a dummy svc_rdma_recv_ctxt object when sending an RPC Call. The need for svc_rdma_recv_ctxt free list serialization is now completely avoided. Signed-off-by: Chuck Lever --- include/linux/sunrpc/svc_rdma.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'include/linux') diff --git a/include/linux/sunrpc/svc_rdma.h b/include/linux/sunrpc/svc_rdma.h index a5ee0af2a310..4ac32895a058 100644 --- a/include/linux/sunrpc/svc_rdma.h +++ b/include/linux/sunrpc/svc_rdma.h @@ -200,7 +200,8 @@ extern int svc_rdma_send(struct svcxprt_rdma *rdma, struct svc_rdma_send_ctxt *ctxt); extern int svc_rdma_map_reply_msg(struct svcxprt_rdma *rdma, struct svc_rdma_send_ctxt *sctxt, - const struct svc_rdma_recv_ctxt *rctxt, + const struct svc_rdma_pcl *write_pcl, + const struct svc_rdma_pcl *reply_pcl, const struct xdr_buf *xdr); extern void svc_rdma_send_error_msg(struct svcxprt_rdma *rdma, struct svc_rdma_send_ctxt *sctxt, -- cgit v1.2.3