NFSD: narrow nfsd_mutex protection in nfsd thread

There is nothing happening in the start of nfsd() that requires
protection by the mutex, so don't take it until shutting down the thread
- which does still require protection - but only for nfsd_put().
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfssvc.c

@@ -932,9 +932,6 @@ nfsd(void *vrqstp)
 	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
 	int err;
 
-	/* Lock module and set up kernel thread */
-	mutex_lock(&nfsd_mutex);
-
 	/* At this point, the thread shares current->fs
 	 * with the init process. We need to create files with the
 	 * umask as defined by the client instead of init's umask. */
@@ -954,7 +951,6 @@ nfsd(void *vrqstp)
 	allow_signal(SIGINT);
 	allow_signal(SIGQUIT);
 
-	mutex_unlock(&nfsd_mutex);
 	atomic_inc(&nfsdstats.th_cnt);
 
 	set_freezable();
@@ -983,7 +979,6 @@ nfsd(void *vrqstp)
 	flush_signals(current);
 
 	atomic_dec(&nfsdstats.th_cnt);
-	mutex_lock(&nfsd_mutex);
 
 out:
 	/* Take an extra ref so that the svc_put in svc_exit_thread()
@@ -995,10 +990,11 @@ nfsd(void *vrqstp)
 	svc_exit_thread(rqstp);
 
 	/* Now if needed we call svc_destroy in appropriate context */
+	mutex_lock(&nfsd_mutex);
 	nfsd_put(net);
+	mutex_unlock(&nfsd_mutex);
 
 	/* Release module */
-	mutex_unlock(&nfsd_mutex);
 	module_put_and_exit(0);
 	return 0;
 }