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Subjectmem-hotplug + ksm make lockdep warning
Date
Hi Hugh,

commit 62b61f611e(ksm: memory hotremove migration only) makes following
lockdep warnings. Is this intentional?

More detail: current lockdep hieralcy is here.

memory_notify
offline_pages
lock_system_sleep();
mutex_lock(&pm_mutex);
memory_notify(MEM_GOING_OFFLINE)
__blocking_notifier_call_chain
down_read(memory_chain.rwsem)
ksm_memory_callback()
mutex_lock(&ksm_thread_mutex); // memory_chain.rmsem -> ksm_thread_mutex order
up_read(memory_chain.rwsem)
memory_notify(MEM_OFFLINE)
__blocking_notifier_call_chain
down_read(memory_chain.rwsem) // ksm_thread_mutex -> memory_chain.rmsem order
ksm_memory_callback()
mutex_unlock(&ksm_thread_mutex);
up_read(memory_chain.rwsem)
unlock_system_sleep();
mutex_unlock(&pm_mutex);

So, I think pm_mutex protect ABBA deadlock. but it exist only when
CONFIG_HIBERNATION=y. IOW, this code is not correct generically. Am I
missing something?

Thanks.



=======================================================
[ INFO: possible circular locking dependency detected ]
2.6.36-rc7-mm1+ #148
-------------------------------------------------------
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

other info that might help us debug this:

5 locks held by bash/1621:
#0: (&buffer->mutex){+.+.+.}, at: [<ffffffff811bc074>] sysfs_write_file+0x44/0x170
#1: (s_active#110){.+.+.+}, at: [<ffffffff811bc0fd>] sysfs_write_file+0xcd/0x170
#2: (&mem->state_mutex){+.+.+.}, at: [<ffffffff813af478>] memory_block_change_state+0x38/0x1a0
#3: (pm_mutex){+.+.+.}, at: [<ffffffff81141ad9>] remove_memory+0x129/0x5f0
#4: (ksm_thread_mutex){+.+.+.}, at: [<ffffffff8113a3aa>] ksm_memory_callback+0x3a/0xc0

stack backtrace:
Pid: 1621, comm: bash Not tainted 2.6.36-rc7-mm1+ #148
Call Trace:
[<ffffffff81088b5b>] print_circular_bug+0xeb/0xf0
[<ffffffff8108b5ba>] __lock_acquire+0x155a/0x1600
[<ffffffff8103a1f9>] ? finish_task_switch+0x79/0xe0
[<ffffffff815049a9>] ? schedule+0x419/0xa80
[<ffffffff8108b70a>] lock_acquire+0xaa/0x140
[<ffffffff81079339>] ? __blocking_notifier_call_chain+0x69/0xc0
[<ffffffff81506601>] down_read+0x51/0xa0
[<ffffffff81079339>] ? __blocking_notifier_call_chain+0x69/0xc0
[<ffffffff81079339>] __blocking_notifier_call_chain+0x69/0xc0
[<ffffffff81110f06>] ? next_online_pgdat+0x26/0x50
[<ffffffff810793a6>] blocking_notifier_call_chain+0x16/0x20
[<ffffffff813afbfb>] memory_notify+0x1b/0x20
[<ffffffff81141f1e>] remove_memory+0x56e/0x5f0
[<ffffffff8108ba98>] ? lock_release_non_nested+0x2f8/0x3a0
[<ffffffff813af53d>] memory_block_change_state+0xfd/0x1a0
[<ffffffff8111705c>] ? might_fault+0x5c/0xb0
[<ffffffff813afd62>] store_mem_state+0xe2/0xf0
[<ffffffff811bc0fd>] ? sysfs_write_file+0xcd/0x170
[<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



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