SUNRPC: close backchannel before destroying callback service

A backchannel receive can complete a request while the NFS callback
service is being torn down.  xprt_complete_bc_request() removes the
request from bc_pa_list, drops bc_alloc_count, marks the request in use,
and then asks xprt_enqueue_bc_request() to hand it to the callback
service.

If teardown has already cleared xprt->bc_serv, xprt_enqueue_bc_request()
currently returns without enqueueing or freeing the committed request.
The xprt_get() taken on entry is leaked as well.  If the producer wins
the race before bc_serv is cleared, it can also enqueue onto sv_cb_list
after nfs_callback_down() has stopped the callback threads, leaving the
request linked to a svc_serv that is about to be freed.

Close the producer side before callback threads are stopped.  Add
xprt_svc_shutdown_bc() to clear xprt->bc_serv under bc_pa_lock, and call
it on callback shutdown and callback-start failure before stopping the
service threads.  Requests that lose the NULL transition in
xprt_enqueue_bc_request() are released through the normal backchannel
free path after balancing bc_slot_count.  Finally, drain any remaining
sv_cb_list requests after the callback threads have stopped and before
svc_destroy() frees the service.

Fixes: 441244d4273a ("SUNRPC: cleanup common code in backchannel request")
Fixes: 9e9fdd0ad0fb ("NFSv4.1: protect destroying and nullifying bc_serv structure")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260528-tier2-v1-6-d026a1415e0b@oracle.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/sunrpc/bc_xprt.h b/include/linux/sunrpc/bc_xprt.h
index 98939cb664cf..59d0cc889beb 100644
--- a/include/linux/sunrpc/bc_xprt.h
+++ b/include/linux/sunrpc/bc_xprt.h
@@ -32,6 +32,7 @@ int xprt_setup_bc(struct rpc_xprt *xprt, unsigned int min_reqs);
 void xprt_destroy_bc(struct rpc_xprt *xprt, unsigned int max_reqs);
 void xprt_free_bc_rqst(struct rpc_rqst *req);
 unsigned int xprt_bc_max_slots(struct rpc_xprt *xprt);
+void xprt_svc_shutdown_bc(struct rpc_xprt *xprt);
 void xprt_svc_destroy_nullify_bc(struct rpc_xprt *xprt, struct svc_serv **serv);
 
 /*
@@ -71,6 +72,10 @@ static inline void xprt_free_bc_request(struct rpc_rqst *req)
 {
 }
 
+static inline void xprt_svc_shutdown_bc(struct rpc_xprt *xprt)
+{
+}
+
 static inline void xprt_svc_destroy_nullify_bc(struct rpc_xprt *xprt, struct svc_serv **serv)
 {
 	svc_destroy(serv);