I've dug into this a little more. Posting these two patches just
as conversation pieces.
The spinning NULL task at umount time, it turns out, is the kernel's
attempt to send GSS DESTROY_CONTEXT. This NULL is triggered just
as the RPC client has killed all tasks. The spinning rpc_task will
run until a soft timeout occurs, and then the umount unhangs.
Maybe set a shutdown flag in the client, and cause early FSM steps
to fail if this flag is set? Or should the client skip context
destruction if this flag is set?
Any other thoughts?
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Chuck Lever (2):
SUNRPC: Set SOFTCONN when destroying GSS contexts
sunrpc: Ensure signalled RPC tasks exit
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Chuck Lever
