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SubjectRe: [stable-3.10.y] possible unsafe locking warning
Hi Johannes,
Sorry for late.
The patch works well with stable kernel-3.10.y, the warning is gone.
Thank you and Tejun very much.:)

Regards,
Gu
On 06/05/2014 09:24 PM, Johannes Weiner wrote:

> Hi,
>
> [cc'ing Andrew and linux-mm for patch review and inclusion]
>
> On Thu, Jun 05, 2014 at 01:44:38PM +0800, Gu Zheng wrote:
>> Hi Tejun,
>> Sorry for late replay.
>> On 05/28/2014 11:48 PM, Tejun Heo wrote:
>>
>>> (cc'ing Johannes for mm-foo)
>>>
>>> Hello,
>>>
>>> On Wed, May 28, 2014 at 06:06:34PM +0800, Gu Zheng wrote:
>>>> [ 2457.683370] inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-R} usage.
>>>> [ 2457.761540] kswapd2/1151 [HC0[0]:SC0[0]:HE1:SE1] takes:
>>>> [ 2457.824102] (&sig->group_rwsem){+++++?}, at: [<ffffffff81071864>] exit_signals+0x24/0x130
>>>> [ 2457.923538] {RECLAIM_FS-ON-W} state was registered at:
>>>> [ 2457.985055] [<ffffffff810bfc99>] mark_held_locks+0xb9/0x140
>>>> [ 2458.053976] [<ffffffff810c1e3a>] lockdep_trace_alloc+0x7a/0xe0
>>>> [ 2458.126015] [<ffffffff81194f47>] kmem_cache_alloc_trace+0x37/0x240
>>>> [ 2458.202214] [<ffffffff812c6e89>] flex_array_alloc+0x99/0x1a0
>>>> [ 2458.272175] [<ffffffff810da563>] cgroup_attach_task+0x63/0x430
>>>> [ 2458.344214] [<ffffffff810dcca0>] attach_task_by_pid+0x210/0x280
>>>> [ 2458.417294] [<ffffffff810dcd26>] cgroup_procs_write+0x16/0x20
>>>> [ 2458.488287] [<ffffffff810d8410>] cgroup_file_write+0x120/0x2c0
>>>> [ 2458.560320] [<ffffffff811b21a0>] vfs_write+0xc0/0x1f0
>>>> [ 2458.622994] [<ffffffff811b2bac>] SyS_write+0x4c/0xa0
>>>> [ 2458.684618] [<ffffffff815ec3c0>] tracesys+0xdd/0xe2
>>>> [ 2458.745214] irq event stamp: 49
>>>> [ 2458.782794] hardirqs last enabled at (49): [<ffffffff815e2b56>] _raw_spin_unlock_irqrestore+0x36/0x70
>>>> [ 2458.894388] hardirqs last disabled at (48): [<ffffffff815e337b>] _raw_spin_lock_irqsave+0x2b/0xa0
>>>> [ 2459.000771] softirqs last enabled at (0): [<ffffffff81059247>] copy_process.part.24+0x627/0x15f0
>>>> [ 2459.107161] softirqs last disabled at (0): [< (null)>] (null)
>>>> [ 2459.195852]
>>>> [ 2459.195852] other info that might help us debug this:
>>>> [ 2459.274024] Possible unsafe locking scenario:
>>>> [ 2459.274024]
>>>> [ 2459.344911] CPU0
>>>> [ 2459.374161] ----
>>>> [ 2459.403408] lock(&sig->group_rwsem);
>>>> [ 2459.448490] <Interrupt>
>>>> [ 2459.479825] lock(&sig->group_rwsem);
>>>> [ 2459.526979]
>>>> [ 2459.526979] *** DEADLOCK ***
>>>> [ 2459.526979]
>>>> [ 2459.597866] no locks held by kswapd2/1151.
>>>> [ 2459.646896]
>>>> [ 2459.646896] stack backtrace:
>>>> [ 2459.699049] CPU: 30 PID: 1151 Comm: kswapd2 Not tainted 3.10.39+ #4
>>>> [ 2459.774098] Hardware name: FUJITSU PRIMEQUEST2800E/SB, BIOS PRIMEQUEST 2000 Series BIOS Version 01.48 05/07/2014
>>>> [ 2459.895983] ffffffff82284bf0 ffff88085856bbf8 ffffffff815dbcf6 ffff88085856bc48
>>>> [ 2459.985003] ffffffff815d67c6 0000000000000000 ffff880800000001 ffff880800000001
>>>> [ 2460.074024] 000000000000000a ffff88085edc9600 ffffffff810be0e0 0000000000000009
>>>> [ 2460.163087] Call Trace:
>>>> [ 2460.192345] [<ffffffff815dbcf6>] dump_stack+0x19/0x1b
>>>> [ 2460.253874] [<ffffffff815d67c6>] print_usage_bug+0x1f7/0x208
>>>> [ 2460.399807] [<ffffffff810bfb5d>] mark_lock+0x21d/0x2a0
>>>> [ 2460.462369] [<ffffffff810c076a>] __lock_acquire+0x52a/0xb60
>>>> [ 2460.735516] [<ffffffff810c1592>] lock_acquire+0xa2/0x140
>>>> [ 2460.935691] [<ffffffff815e01e1>] down_read+0x51/0xa0
>>>> [ 2461.062888] [<ffffffff81071864>] exit_signals+0x24/0x130
>>>> [ 2461.127536] [<ffffffff81060d55>] do_exit+0xb5/0xa50
>>>> [ 2461.320433] [<ffffffff8108303b>] kthread+0xdb/0x100
>>>> [ 2461.532049] [<ffffffff815ec0ec>] ret_from_fork+0x7c/0xb0
>>>
>>> The lockdep warning is about threadgroup_lock being grabbed by kswapd
>>> which is depended upon during memory reclaim when the lock may be held
>>> by tasks which may wait on memory reclaim. From the backtrace, it
>>> looks like the right thing to do is marking the kswapd that it's no
>>> longer a memory reclaimer once before it starts exiting.
>
> Yeah, that makes sense. In fact, we can reset *all* the
> reclaim-specific per-task states the second it stops performing
> reclaim work.
>
>>>> And when reference to the related code(kernel-3.10.y), it seems that cgroup_attach_task(thread-2,
>>>> attach kswapd) trigger kswapd(reclaim memory?) when trying to alloc memory(flex_array_alloc) under
>>>> the protection of sig->group_rwsem, but meanwhile the kswapd(thread-1) is in the exit routine
>>>> (because it was marked SHOULD STOP when offline pages completed), which needs to acquire
>>>> sig->group_rwsem in exit_signals(), so the deadlock occurs.
>>>>
>>>> thread-1 | thread-2
>>>> |
>>>> __offline_pages(): | system_call_fastpath()
>>>> |-> kswapd_stop(node); | |-> ......
>>>> |-> kthread_stop(kswapd) | |-> cgroup_file_write()
>>>> |-> set_bit(KTHREAD_SHOULD_STOP, &kthread->flags); | |-> ......
>>>> |-> wake_up_process(k) | |-> attach_task_by_pid()
>>>> | | |-> threadgroup_lock(tsk)
>>>> |<----------| | // Here, got the lock.
>>>> |-> kswapd() | |-> ...
>>>> |-> if (kthread_should_stop()) | |-> cgroup_attach_task()
>>>> return; | |-> flex_array_alloc()
>>>> | | |-> kzalloc()
>>>> |<----------| | |-> wait for kswapd to reclaim memory
>>>> |-> kthread() |
>>>> |-> do_exit(ret) |
>>>> |-> exit_signals() |
>>>> |-> threadgroup_change_begin(tsk) |
>>>> |-> down_read(&tsk->signal->group_rwsem) |
>>>> // Here, acquire the lock.
>>>>
>>>> If my analysis is correct, the latest kernel may have the same issue, though the flex_array was replaced
>>>> by list, but we still need to alloc memory(e.g. in find_css_set()), so the race may still occur.
>>>> Any comments about this? If I missed something, please correct me.:)
>>>
>>> Not sure whether this can actually happen but if so the right fix
>>> would be making thread-2 not wait for kswapd which is exiting and can
>>> no longer serve as memory reclaimer.
>
> There is never a direct wait for a specific kswapd thread in the
> waitqueue sense. The allocator wakes up the kswapds for all nodes
> allowed in the allocation, then retries the allocation a few times in
> the hope that kswapd does something before entering reclaim itself.
>
> How far back do we need this in stable?
>
> ---
>
>>From c3d76e3c208bc90b64b804ffefa114b920cab47e Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@cmpxchg.org>
> Date: Thu, 5 Jun 2014 08:37:01 -0400
> Subject: [patch] mm: vmscan: clear kswapd's special reclaim powers before
> exiting
>
> When kswapd exits, it can end up taking locks that were previously
> held by allocating tasks while they waited for reclaim. Lockdep
> currently warns about this:
>
> On Wed, May 28, 2014 at 06:06:34PM +0800, Gu Zheng wrote:
>> [ 2457.683370] inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-R} usage.
>> [ 2457.761540] kswapd2/1151 [HC0[0]:SC0[0]:HE1:SE1] takes:
>> [ 2457.824102] (&sig->group_rwsem){+++++?}, at: [<ffffffff81071864>] exit_signals+0x24/0x130
>> [ 2457.923538] {RECLAIM_FS-ON-W} state was registered at:
>> [ 2457.985055] [<ffffffff810bfc99>] mark_held_locks+0xb9/0x140
>> [ 2458.053976] [<ffffffff810c1e3a>] lockdep_trace_alloc+0x7a/0xe0
>> [ 2458.126015] [<ffffffff81194f47>] kmem_cache_alloc_trace+0x37/0x240
>> [ 2458.202214] [<ffffffff812c6e89>] flex_array_alloc+0x99/0x1a0
>> [ 2458.272175] [<ffffffff810da563>] cgroup_attach_task+0x63/0x430
>> [ 2458.344214] [<ffffffff810dcca0>] attach_task_by_pid+0x210/0x280
>> [ 2458.417294] [<ffffffff810dcd26>] cgroup_procs_write+0x16/0x20
>> [ 2458.488287] [<ffffffff810d8410>] cgroup_file_write+0x120/0x2c0
>> [ 2458.560320] [<ffffffff811b21a0>] vfs_write+0xc0/0x1f0
>> [ 2458.622994] [<ffffffff811b2bac>] SyS_write+0x4c/0xa0
>> [ 2458.684618] [<ffffffff815ec3c0>] tracesys+0xdd/0xe2
>> [ 2458.745214] irq event stamp: 49
>> [ 2458.782794] hardirqs last enabled at (49): [<ffffffff815e2b56>] _raw_spin_unlock_irqrestore+0x36/0x70
>> [ 2458.894388] hardirqs last disabled at (48): [<ffffffff815e337b>] _raw_spin_lock_irqsave+0x2b/0xa0
>> [ 2459.000771] softirqs last enabled at (0): [<ffffffff81059247>] copy_process.part.24+0x627/0x15f0
>> [ 2459.107161] softirqs last disabled at (0): [< (null)>] (null)
>> [ 2459.195852]
>> [ 2459.195852] other info that might help us debug this:
>> [ 2459.274024] Possible unsafe locking scenario:
>> [ 2459.274024]
>> [ 2459.344911] CPU0
>> [ 2459.374161] ----
>> [ 2459.403408] lock(&sig->group_rwsem);
>> [ 2459.448490] <Interrupt>
>> [ 2459.479825] lock(&sig->group_rwsem);
>> [ 2459.526979]
>> [ 2459.526979] *** DEADLOCK ***
>> [ 2459.526979]
>> [ 2459.597866] no locks held by kswapd2/1151.
>> [ 2459.646896]
>> [ 2459.646896] stack backtrace:
>> [ 2459.699049] CPU: 30 PID: 1151 Comm: kswapd2 Not tainted 3.10.39+ #4
>> [ 2459.774098] Hardware name: FUJITSU PRIMEQUEST2800E/SB, BIOS PRIMEQUEST 2000 Series BIOS Version 01.48 05/07/2014
>> [ 2459.895983] ffffffff82284bf0 ffff88085856bbf8 ffffffff815dbcf6 ffff88085856bc48
>> [ 2459.985003] ffffffff815d67c6 0000000000000000 ffff880800000001 ffff880800000001
>> [ 2460.074024] 000000000000000a ffff88085edc9600 ffffffff810be0e0 0000000000000009
>> [ 2460.163087] Call Trace:
>> [ 2460.192345] [<ffffffff815dbcf6>] dump_stack+0x19/0x1b
>> [ 2460.253874] [<ffffffff815d67c6>] print_usage_bug+0x1f7/0x208
>> [ 2460.399807] [<ffffffff810bfb5d>] mark_lock+0x21d/0x2a0
>> [ 2460.462369] [<ffffffff810c076a>] __lock_acquire+0x52a/0xb60
>> [ 2460.735516] [<ffffffff810c1592>] lock_acquire+0xa2/0x140
>> [ 2460.935691] [<ffffffff815e01e1>] down_read+0x51/0xa0
>> [ 2461.062888] [<ffffffff81071864>] exit_signals+0x24/0x130
>> [ 2461.127536] [<ffffffff81060d55>] do_exit+0xb5/0xa50
>> [ 2461.320433] [<ffffffff8108303b>] kthread+0xdb/0x100
>> [ 2461.532049] [<ffffffff815ec0ec>] ret_from_fork+0x7c/0xb0
>
> This is because the kswapd thread is still marked as a reclaimer at
> the time of exit. But because it is exiting, nobody is actually
> waiting on it to make reclaim progress anymore, and it's nothing but a
> regular thread at this point. Be tidy and strip it of all its powers
> (PF_MEMALLOC, PF_SWAPWRITE, PF_KSWAPD, and the lockdep reclaim state)
> before returning from the thread function.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
> mm/vmscan.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9a63d13739a6..4ac2eab860d2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3425,7 +3425,10 @@ static int kswapd(void *p)
> }
> }
>
> + tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
> current->reclaim_state = NULL;
> + lockdep_clear_current_reclaim_state();
> +
> return 0;
> }
>




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