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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Oct 03, 2012 at 08:25:28PM +0530, Srivatsa S. Bhat wrote:
> 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.

Good point!!!  False positive it is!

							Thanx, Paul

> > 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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