ksm: annotate ksm_thread_mutex is no deadlock source

commit 62b61f611e ("ksm: memory hotremove migration only") caused the
following new lockdep warning.

  =======================================================
  [ INFO: possible circular locking dependency detected ]
  -------------------------------------------------------
  bash/1621 is trying to acquire lock:
   ((memory_chain).rwsem){.+.+.+}, at: [<ffffffff81079339>]
  __blocking_notifier_call_chain+0x69/0xc0

  but task is already holding lock:
   (ksm_thread_mutex){+.+.+.}, at: [<ffffffff8113a3aa>]
  ksm_memory_callback+0x3a/0xc0

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> #1 (ksm_thread_mutex){+.+.+.}:
       [<ffffffff8108b70a>] lock_acquire+0xaa/0x140
       [<ffffffff81505d74>] __mutex_lock_common+0x44/0x3f0
       [<ffffffff81506228>] mutex_lock_nested+0x48/0x60
       [<ffffffff8113a3aa>] ksm_memory_callback+0x3a/0xc0
       [<ffffffff8150c21c>] notifier_call_chain+0x8c/0xe0
       [<ffffffff8107934e>] __blocking_notifier_call_chain+0x7e/0xc0
       [<ffffffff810793a6>] blocking_notifier_call_chain+0x16/0x20
       [<ffffffff813afbfb>] memory_notify+0x1b/0x20
       [<ffffffff81141b7c>] remove_memory+0x1cc/0x5f0
       [<ffffffff813af53d>] memory_block_change_state+0xfd/0x1a0
       [<ffffffff813afd62>] store_mem_state+0xe2/0xf0
       [<ffffffff813a0bb0>] sysdev_store+0x20/0x30
       [<ffffffff811bc116>] sysfs_write_file+0xe6/0x170
       [<ffffffff8114f398>] vfs_write+0xc8/0x190
       [<ffffffff8114fc14>] sys_write+0x54/0x90
       [<ffffffff810028b2>] system_call_fastpath+0x16/0x1b

  -> #0 ((memory_chain).rwsem){.+.+.+}:
       [<ffffffff8108b5ba>] __lock_acquire+0x155a/0x1600
       [<ffffffff8108b70a>] lock_acquire+0xaa/0x140
       [<ffffffff81506601>] down_read+0x51/0xa0
       [<ffffffff81079339>] __blocking_notifier_call_chain+0x69/0xc0
       [<ffffffff810793a6>] blocking_notifier_call_chain+0x16/0x20
       [<ffffffff813afbfb>] memory_notify+0x1b/0x20
       [<ffffffff81141f1e>] remove_memory+0x56e/0x5f0
       [<ffffffff813af53d>] memory_block_change_state+0xfd/0x1a0
       [<ffffffff813afd62>] store_mem_state+0xe2/0xf0
       [<ffffffff813a0bb0>] sysdev_store+0x20/0x30
       [<ffffffff811bc116>] sysfs_write_file+0xe6/0x170
       [<ffffffff8114f398>] vfs_write+0xc8/0x190
       [<ffffffff8114fc14>] sys_write+0x54/0x90
       [<ffffffff810028b2>] system_call_fastpath+0x16/0x1b

But it's a false positive.  Both memory_chain.rwsem and ksm_thread_mutex
have an outer lock (mem_hotplug_mutex).  So they cannot deadlock.

Thus, This patch annotate ksm_thread_mutex is not deadlock source.

[akpm@linux-foundation.org: update comment, from Hugh]
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/ksm.c

@@ -1724,8 +1724,13 @@ static int ksm_memory_callback(struct notifier_block *self,
 		/*
 		 * Keep it very simple for now: just lock out ksmd and
 		 * MADV_UNMERGEABLE while any memory is going offline.
+		 * mutex_lock_nested() is necessary because lockdep was alarmed
+		 * that here we take ksm_thread_mutex inside notifier chain
+		 * mutex, and later take notifier chain mutex inside
+		 * ksm_thread_mutex to unlock it.   But that's safe because both
+		 * are inside mem_hotplug_mutex.
 		 */
-		mutex_lock(&ksm_thread_mutex);
+		mutex_lock_nested(&ksm_thread_mutex, SINGLE_DEPTH_NESTING);
 		break;
 
 	case MEM_OFFLINE: