svcrdma: Eliminate allocation of recv_ctxt objects in backchannel

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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 <chuck.lever@oracle.com>

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This commit is contained in:

Chuck Lever 2023-11-21 11:40:13 -05:00

parent 52e8910075

commit b541dd554b

3 changed files with 23 additions and 22 deletions

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3

include/linux/sunrpc/svc_rdma.h

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11

net/sunrpc/xprtrdma/svc_rdma_backchannel.c

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31

net/sunrpc/xprtrdma/svc_rdma_sendto.c

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