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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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.