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Subject[PATCH 2/2] ksm: annotate ksm_thread_mutex is no deadlock source
Date

commit 62b61f611e(ksm: memory hotremove migration only) made 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 false positive. Both memory_chain.rwsem and ksm_thread_mutex
have outer lock (mem_hotplug_mutex). then, they can't make deadlock.

Thus, This patch annotate ksm_thread_mutex is not deadlock source.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
mm/ksm.c | 4 +++-
1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 65ab5c7..5aa4900 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1724,8 +1724,10 @@ 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 to tell that
+ * ksm_thread_mutex is not unlocked here intentionally.
*/
- mutex_lock(&ksm_thread_mutex);
+ mutex_lock_nested(&ksm_thread_mutex, SINGLE_DEPTH_NESTING);
break;

case MEM_OFFLINE:
--
1.6.5.2




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