lockd: fix list corruption on lockd restart

If lockd is signalled soon enough after restart then locks_start_grace()
will try to re-add an entry to a list and trigger a lock corruption
warning.

Thanks to Wang Chen for the problem report and diagnosis.

WARNING: at lib/list_debug.c:26 __list_add+0x27/0x5c()
...
list_add corruption. next->prev should be prev (ef8fe958), but was ef8ff128.  (next=ef8ff128).
...
Pid: 23062, comm: lockd Tainted: G        W  2.6.30-rc2 #3
Call Trace:
[<c042d5b5>] warn_slowpath+0x71/0xa0
[<c0422a96>] ? update_curr+0x11d/0x125
[<c044b12d>] ? trace_hardirqs_on_caller+0x18/0x150
[<c044b270>] ? trace_hardirqs_on+0xb/0xd
[<c051c61a>] ? _raw_spin_lock+0x53/0xfa
[<c051c89f>] __list_add+0x27/0x5c
[<ef8f6daa>] locks_start_grace+0x22/0x30 [lockd]
[<ef8f34da>] set_grace_period+0x39/0x53 [lockd]
[<c06b8921>] ? lock_kernel+0x1c/0x28
[<ef8f3558>] lockd+0x64/0x164 [lockd]
[<c044b12d>] ? trace_hardirqs_on_caller+0x18/0x150
[<c04227b0>] ? complete+0x34/0x3e
[<ef8f34f4>] ? lockd+0x0/0x164 [lockd]
[<ef8f34f4>] ? lockd+0x0/0x164 [lockd]
[<c043dd42>] kthread+0x45/0x6b
[<c043dcfd>] ? kthread+0x0/0x6b
[<c0403c23>] kernel_thread_helper+0x7/0x10
Reported-by: NWang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Cc: stable@kernel.org

fs/lockd/svc.c

@@ -104,6 +104,16 @@ static void set_grace_period(void)
 	schedule_delayed_work(&grace_period_end, grace_period);
 }
 
+static void restart_grace(void)
+{
+	if (nlmsvc_ops) {
+		cancel_delayed_work_sync(&grace_period_end);
+		locks_end_grace(&lockd_manager);
+		nlmsvc_invalidate_all();
+		set_grace_period();
+	}
+}
+
 /*
  * This is the lockd kernel thread
  */
@@ -149,10 +159,7 @@ lockd(void *vrqstp)
 
 		if (signalled()) {
 			flush_signals(current);
-			if (nlmsvc_ops) {
-				nlmsvc_invalidate_all();
-				set_grace_period();
-			}
+			restart_grace();
 			continue;
 		}