Message-ID: <20120716144951.1f1f574a@thinkpad>
Date:	Mon, 16 Jul 2012 14:49:51 +0200
From:	Gerald Schaefer <gerald.schaefer@...ibm.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Martin Schwidefsky <martin.schwidefsky@...ibm.com>,
	Heiko Carstens <h.carstens@...ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: ksm/memory hotplug: lockdep warning for ksm_thread_mutex vs.
 (memory_chain).rwsem

On Thu, 2 Feb 2012 18:00:45 -0500
KOSAKI Motohiro <kosaki.motohiro@...il.com> wrote:

> 2012/2/2 Gerald Schaefer <gerald.schaefer@...ibm.com>:
> > Setting a memory block offline triggers the following lockdep
> > warning. This looks exactly like the issue reported by Kosaki
> > Motohiro in https://lkml.org/lkml/2010/10/25/110. Seems like the
> > resulting commit a0b0f58cdd did not fix the lockdep warning. I'm
> > able to reproduce it with current 3.3.0-rc2 as well as
> > 2.6.37-rc4-00147-ga0b0f58.
> >
> > I'm not familiar with lockdep annotations, but I tried using
> > down_read_nested() for (memory_chain).rwsem, similar to the
> > mutex_lock_nested() which was introduced for ksm_thread_mutex, but
> > that didn't help.
> 
> Heh, interesting. Simple question, do you have any user visible buggy
> behavior? or just false positive warn issue?
> 
> *_nested() is just hacky trick. so, any change may break their lie.
> Anyway I'd like to dig this one. thanks for reporting.

Hi,

any news on this? I'm still getting test reports about the lockdep
warning: the problem is still present in 3.5.0-rc7 and it still looks
like a false-positive to me (both locks inside mem_hotplug_mutex, so
there can't be a deadlock, see also comment in mm/ksm.c). Any ideas how
to convince lockdep of that, so that we can run memory hotplug tests
again with lockdep enabled?

======================================================
[ INFO: possible circular locking dependency detected ]
3.5.0-rc7 #40 Not tainted
-------------------------------------------------------
sh/698 is trying to acquire lock:
 ((memory_chain).rwsem){.+.+.+}, at: [<0000000000165372>] __blocking_notifier_call_chain+0x5e/0xe0

but task is already holding lock:
 (ksm_thread_mutex/1){+.+.+.}, at: [<000000000026a654>] ksm_memory_callback+0x48/0xd8

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (ksm_thread_mutex/1){+.+.+.}:
       [<00000000001a5b42>] __lock_acquire+0x3f6/0xb28
       [<00000000001a69d2>] lock_acquire+0xbe/0x250
       [<000000000064cefc>] mutex_lock_nested+0x98/0x3f4
       [<000000000026a654>] ksm_memory_callback+0x48/0xd8
       [<0000000000653af4>] notifier_call_chain+0x8c/0x174
       [<0000000000165388>] __blocking_notifier_call_chain+0x74/0xe0
       [<000000000016541e>] blocking_notifier_call_chain+0x2a/0x3c
       [<0000000000638cb4>] offline_pages.constprop.1+0x17c/0x740
       [<00000000004b5326>] memory_block_change_state+0x2aa/0x328
       [<00000000004b545e>] store_mem_state+0xba/0xf0
       [<000000000030b5f2>] sysfs_write_file+0xf6/0x1a8
       [<0000000000283a3a>] vfs_write+0x9a/0x184
       [<0000000000283d94>] SyS_write+0x58/0x94
       [<00000000006514f4>] sysc_noemu+0x22/0x28
       [<000003fffd5aa3e8>] 0x3fffd5aa3e8

-> #0 ((memory_chain).rwsem){.+.+.+}:
       [<00000000001a22bc>] validate_chain+0x880/0x1154
       [<00000000001a5b42>] __lock_acquire+0x3f6/0xb28
       [<00000000001a69d2>] lock_acquire+0xbe/0x250
       [<000000000064d8a6>] down_read+0x66/0xdc
       [<0000000000165372>] __blocking_notifier_call_chain+0x5e/0xe0
       [<000000000016541e>] blocking_notifier_call_chain+0x2a/0x3c
       [<0000000000638d00>] offline_pages.constprop.1+0x1c8/0x740
       [<00000000004b5326>] memory_block_change_state+0x2aa/0x328
       [<00000000004b545e>] store_mem_state+0xba/0xf0
       [<000000000030b5f2>] sysfs_write_file+0xf6/0x1a8
       [<0000000000283a3a>] vfs_write+0x9a/0x184
       [<0000000000283d94>] SyS_write+0x58/0x94
       [<00000000006514f4>] sysc_noemu+0x22/0x28
       [<000003fffd5aa3e8>] 0x3fffd5aa3e8

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(ksm_thread_mutex/1);
                               lock((memory_chain).rwsem);
                               lock(ksm_thread_mutex/1);
  lock((memory_chain).rwsem);

 *** DEADLOCK ***

6 locks held by sh/698:
 #0:  (&buffer->mutex){+.+.+.}, at: [<000000000030b546>] sysfs_write_file+0x4a/0x1a8
 #1:  (s_active#31){.+.+.+}, at: [<000000000030b5ce>] sysfs_write_file+0xd2/0x1a8
 #2:  (&mem->state_mutex){+.+.+.}, at: [<00000000004b50be>] memory_block_change_state+0x42/0x328
 #3:  (mem_hotplug_mutex){+.+.+.}, at: [<0000000000275324>] lock_memory_hotplug+0x2c/0x4c
 #4:  (pm_mutex){+.+.+.}, at: [<0000000000638c22>] offline_pages.constprop.1+0xea/0x740
 #5:  (ksm_thread_mutex/1){+.+.+.}, at: [<000000000026a654>] ksm_memory_callback+0x48/0xd8

stack backtrace:
CPU: 1 Not tainted 3.5.0-rc7 #40
Process sh (pid: 698, task: 00000000d6b74850, ksp: 00000000d71c7ac0)
       00000000d71c78d8 00000000d71c78e8 0000000000000002 0000000000000000 
       00000000d71c7978 00000000d71c78f0 00000000d71c78f0 00000000001009e0 
       0000000000000000 0000000000000001 000000000000000b 000000000000000b 
       00000000d71c7938 00000000d71c78d8 0000000000000000 0000000000000000 
       0000000000660768 00000000001009e0 00000000d71c78d8 00000000d71c7928 
Call Trace:
([<00000000001008e6>] show_trace+0xee/0x144)
 [<000000000064436e>] print_circular_bug+0x2ee/0x300
 [<00000000001a22bc>] validate_chain+0x880/0x1154
 [<00000000001a5b42>] __lock_acquire+0x3f6/0xb28
 [<00000000001a69d2>] lock_acquire+0xbe/0x250
 [<000000000064d8a6>] down_read+0x66/0xdc
 [<0000000000165372>] __blocking_notifier_call_chain+0x5e/0xe0
 [<000000000016541e>] blocking_notifier_call_chain+0x2a/0x3c
 [<0000000000638d00>] offline_pages.constprop.1+0x1c8/0x740
 [<00000000004b5326>] memory_block_change_state+0x2aa/0x328
 [<00000000004b545e>] store_mem_state+0xba/0xf0
 [<000000000030b5f2>] sysfs_write_file+0xf6/0x1a8
 [<0000000000283a3a>] vfs_write+0x9a/0x184
 [<0000000000283d94>] SyS_write+0x58/0x94
 [<00000000006514f4>] sysc_noemu+0x22/0x28
 [<000003fffd5aa3e8>] 0x3fffd5aa3e8
INFO: lockdep is turned off.

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