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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:441244d427("SUNRPC: cleanup common code in backchannel request") Fixes:9e9fdd0ad0("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>