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. 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); > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>