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

[Paul E. McKenney]

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@xxxxxxx>
> > 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@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
> 
> Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> 
> 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.

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?

							Thanx, Paul

> 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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