NFS: add nfs_sb_deactive_async to avoid deadlock
Use nfs_sb_deactive_async instead of nfs_sb_deactive when in a workqueue context. This avoids a deadlock where rpc_shutdown_client loops forever in a workqueue kworker context, trying to kill all RPC tasks associated with the client, while one or more of these tasks have already been assigned to the same kworker (and will never run rpc_exit_task). This approach is needed because RPC tasks that have already been assigned to a kworker by queue_work cannot be canceled, as explained in the comment for workqueue.c:insert_wq_barrier. Signed-off-by: NWeston Andros Adamson <dros@netapp.com> [Trond: add module_get/put.] Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>
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