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

[Hugh Dickins <hughd@google.com>]

On Wed, 1 Dec 2010, KOSAKI Motohiro wrote:
> 
> 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>

Thank you (I assume it does the job, I've not yet checked).  My only
issue with this is that the comment you added below tells a different
story from the fuller comment you give above.  Maybe change it to:

	* 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.

Acked-by: Hugh Dickins <hughd@google.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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