Re: [PATCH v2] [RFC] mm, slab: release slab_mutex earlier in kmem_cache_destroy()

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 10/03/2012 08:20 PM, Paul E. McKenney wrote:
> On Wed, Oct 03, 2012 at 05:52:26PM +0530, Srivatsa S. Bhat wrote:
>> On 10/03/2012 03:16 PM, Jiri Kosina wrote:
>>> On Wed, 3 Oct 2012, Jiri Kosina wrote:
>>>
>>>> Good question. I believe it should be safe to drop slab_mutex earlier, as 
>>>> cachep has already been unlinked. I am adding slab people and linux-mm to 
>>>> CC (the whole thread on LKML can be found at 
>>>> https://lkml.org/lkml/2012/10/2/296 for reference).
>>>>
>>>> How about the patch below? Pekka, Christoph, please?
>>>
>>> It turns out that lockdep is actually getting this wrong, so the changelog 
>>> in the previous version wasn't accurate.
>>>
>>> Please find patch with updated changelog below. Pekka, Christoph, could 
>>> you please check whether it makes sense to you as well? Thanks.
>>>
>>>
>>>
>>>
>>> From: Jiri Kosina <jkosina@suse.cz>
>>> Subject: [PATCH] mm, slab: release slab_mutex earlier in kmem_cache_destroy()
>>>
>>> Commit 1331e7a1bbe1 ("rcu: Remove _rcu_barrier() dependency on
>>> __stop_machine()") introduced slab_mutex -> cpu_hotplug.lock
>>> dependency through kmem_cache_destroy() -> rcu_barrier() ->
>>> _rcu_barrier() -> get_online_cpus().
>>>
>>> Lockdep thinks that this might actually result in ABBA deadlock,
>>> and reports it as below:
>>>
>>> === [ cut here ] ===
>>>  ======================================================
>>>  [ INFO: possible circular locking dependency detected ]
>>>  3.6.0-rc5-00004-g0d8ee37 #143 Not tainted
>>>  -------------------------------------------------------
>>>  kworker/u:2/40 is trying to acquire lock:
>>>   (rcu_sched_state.barrier_mutex){+.+...}, at: [<ffffffff810f2126>] _rcu_barrier+0x26/0x1e0
>>>
>>>  but task is already holding lock:
>>>   (slab_mutex){+.+.+.}, at: [<ffffffff81176e15>] kmem_cache_destroy+0x45/0xe0
>>>
>>>  which lock already depends on the new lock.
>>>
>>>  the existing dependency chain (in reverse order) is:
>>>
>>>  -> #2 (slab_mutex){+.+.+.}:
>>>         [<ffffffff810ae1e2>] validate_chain+0x632/0x720
>>>         [<ffffffff810ae5d9>] __lock_acquire+0x309/0x530
>>>         [<ffffffff810ae921>] lock_acquire+0x121/0x190
>>>         [<ffffffff8155d4cc>] __mutex_lock_common+0x5c/0x450
>>>         [<ffffffff8155d9ee>] mutex_lock_nested+0x3e/0x50
>>>         [<ffffffff81558cb5>] cpuup_callback+0x2f/0xbe
>>>         [<ffffffff81564b83>] notifier_call_chain+0x93/0x140
>>>         [<ffffffff81076f89>] __raw_notifier_call_chain+0x9/0x10
>>>         [<ffffffff8155719d>] _cpu_up+0xba/0x14e
>>>         [<ffffffff815572ed>] cpu_up+0xbc/0x117
>>>         [<ffffffff81ae05e3>] smp_init+0x6b/0x9f
>>>         [<ffffffff81ac47d6>] kernel_init+0x147/0x1dc
>>>         [<ffffffff8156ab44>] kernel_thread_helper+0x4/0x10
>>>
>>>  -> #1 (cpu_hotplug.lock){+.+.+.}:
>>>         [<ffffffff810ae1e2>] validate_chain+0x632/0x720
>>>         [<ffffffff810ae5d9>] __lock_acquire+0x309/0x530
>>>         [<ffffffff810ae921>] lock_acquire+0x121/0x190
>>>         [<ffffffff8155d4cc>] __mutex_lock_common+0x5c/0x450
>>>         [<ffffffff8155d9ee>] mutex_lock_nested+0x3e/0x50
>>>         [<ffffffff81049197>] get_online_cpus+0x37/0x50
>>>         [<ffffffff810f21bb>] _rcu_barrier+0xbb/0x1e0
>>>         [<ffffffff810f22f0>] rcu_barrier_sched+0x10/0x20
>>>         [<ffffffff810f2309>] rcu_barrier+0x9/0x10
>>>         [<ffffffff8118c129>] deactivate_locked_super+0x49/0x90
>>>         [<ffffffff8118cc01>] deactivate_super+0x61/0x70
>>>         [<ffffffff811aaaa7>] mntput_no_expire+0x127/0x180
>>>         [<ffffffff811ab49e>] sys_umount+0x6e/0xd0
>>>         [<ffffffff81569979>] system_call_fastpath+0x16/0x1b
>>>
>>>  -> #0 (rcu_sched_state.barrier_mutex){+.+...}:
>>>         [<ffffffff810adb4e>] check_prev_add+0x3de/0x440
>>>         [<ffffffff810ae1e2>] validate_chain+0x632/0x720
>>>         [<ffffffff810ae5d9>] __lock_acquire+0x309/0x530
>>>         [<ffffffff810ae921>] lock_acquire+0x121/0x190
>>>         [<ffffffff8155d4cc>] __mutex_lock_common+0x5c/0x450
>>>         [<ffffffff8155d9ee>] mutex_lock_nested+0x3e/0x50
>>>         [<ffffffff810f2126>] _rcu_barrier+0x26/0x1e0
>>>         [<ffffffff810f22f0>] rcu_barrier_sched+0x10/0x20
>>>         [<ffffffff810f2309>] rcu_barrier+0x9/0x10
>>>         [<ffffffff81176ea1>] kmem_cache_destroy+0xd1/0xe0
>>>         [<ffffffffa04c3154>] nf_conntrack_cleanup_net+0xe4/0x110 [nf_conntrack]
>>>         [<ffffffffa04c31aa>] nf_conntrack_cleanup+0x2a/0x70 [nf_conntrack]
>>>         [<ffffffffa04c42ce>] nf_conntrack_net_exit+0x5e/0x80 [nf_conntrack]
>>>         [<ffffffff81454b79>] ops_exit_list+0x39/0x60
>>>         [<ffffffff814551ab>] cleanup_net+0xfb/0x1b0
>>>         [<ffffffff8106917b>] process_one_work+0x26b/0x4c0
>>>         [<ffffffff81069f3e>] worker_thread+0x12e/0x320
>>>         [<ffffffff8106f73e>] kthread+0x9e/0xb0
>>>         [<ffffffff8156ab44>] kernel_thread_helper+0x4/0x10
>>>
>>>  other info that might help us debug this:
>>>
>>>  Chain exists of:
>>>    rcu_sched_state.barrier_mutex --> cpu_hotplug.lock --> slab_mutex
>>>
>>>   Possible unsafe locking scenario:
>>>
>>>         CPU0                    CPU1
>>>         ----                    ----
>>>    lock(slab_mutex);
>>>                                 lock(cpu_hotplug.lock);
>>>                                 lock(slab_mutex);
>>>    lock(rcu_sched_state.barrier_mutex);
>>>
>>>   *** DEADLOCK ***
>>> === [ cut here ] ===
>>>
>>> This is actually a false positive. Lockdep has no way of knowing the fact
>>> that the ABBA can actually never happen, because of special semantics of
>>> cpu_hotplug.refcount and itss handling in cpu_hotplug_begin(); the mutual
>>> exclusion there is not achieved through mutex, but through
>>> cpu_hotplug.refcount.
>>>
>>> The "neither cpu_up() nor cpu_down() will proceed past cpu_hotplug_begin()
>>> until everyone who called get_online_cpus() will call put_online_cpus()"
>>> semantics is totally invisible to lockdep.
>>>
>>> This patch therefore moves the unlock of slab_mutex so that rcu_barrier()
>>> is being called with it unlocked. It has two advantages:
>>>
>>> - it slightly reduces hold time of slab_mutex; as it's used to protect
>>>   the cachep list, it's not necessary to hold it over __kmem_cache_destroy()
>>>   call any more
>>> - it silences the lockdep false positive warning, as it avoids lockdep ever
>>>   learning about slab_mutex -> cpu_hotplug.lock dependency
>>>
>>> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>>> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
>>
>> Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
>>
>> Earlier I was under the wrong impression that all the calltraces that lockdep
>> spewed were reflecting the same point in time. So, sorry about that noise! :-)
>> It is indeed a false-positive and there is no real bug here, and the fix looks
>> good, provided __kmem_cache_destroy() doesn't expect slab_mutex to be held.
> 
> I am not so sure about it being a false positive.  Consider the following
> sequence of events:
> 
> o	Thread A starts a CPU-hotplug operation, acquiring the
> 	hotplug mutex.
> 
> o	Thread B does a kmem_cache_destroy(), acquiring the slab mutex.

This can't happen. Because kmem_cache_destroy() will call get_online_cpus() before
trying to acquire slab mutex. And it sleeps waiting at get_online_cpus() because
the hotplug lock has already been acquired by Thread A.

> 
> o	Thread A reaches the slab CPU-hotplug notifier, but cannot acquire
> 	the slab mutex because Thread B hold it.
> 
> o	Thread B enters rcu_barrier(), but cannot acquire the hotplug
> 	mutex because Thread A holds it.
> 
> So I would argue that lockdep's output was a bit confusing, but that
> the deadlock it flagged is real.  Or am I still missing something?
>

So the key point is, Thread A is a hotplug writer. Thread B becomes a hotplug reader
the moment it calls get_online_cpus(). So they can't co-exist/run together. They will
get serialized. That is, Thread A runs to completion, releases hotplug lock. Only then
thread B gets past get_online_cpus().

Regards,
Srivatsa S. Bhat
  
>> But, I'm also quite surprised that the put_online_cpus() code as it stands today
>> doesn't have any checks for the refcount going negative. I believe that such a
>> check would be valuable to help catch cases where we might end up inadvertently
>> causing an imbalance between get_online_cpus() and put_online_cpus(). I'll post
>> that as a separate patch.
>>
>> Regards,
>> Srivatsa S. Bhat
>>
>>> ---
>>>  mm/slab.c |    2 +-
>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/mm/slab.c b/mm/slab.c
>>> index 1133911..693c7cb 100644
>>> --- a/mm/slab.c
>>> +++ b/mm/slab.c
>>> @@ -2801,12 +2801,12 @@ void kmem_cache_destroy(struct kmem_cache *cachep)
>>>  		put_online_cpus();
>>>  		return;
>>>  	}
>>> +	mutex_unlock(&slab_mutex);
>>>
>>>  	if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU))
>>>  		rcu_barrier();
>>>
>>>  	__kmem_cache_destroy(cachep);
>>> -	mutex_unlock(&slab_mutex);
>>>  	put_online_cpus();
>>>  }
>>>  EXPORT_SYMBOL(kmem_cache_destroy);
>>>
>>

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