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@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. 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); >>> >> -- 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/ . 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