On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote:
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 9c21725..90c3053 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -166,6 +166,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
> > s->refcount--;
> > if (!s->refcount) {
> > list_del(&s->list);
> > + mutex_unlock(&slab_mutex);
> >
> > if (!__kmem_cache_shutdown(s)) {
>
> __kmem_cache_shutdown() calls __cache_shrink(). And __cache_shrink() has this
> comment over it:
> /* Called with slab_mutex held to protect against cpu hotplug */
>
> So, I guess the question is whether to modify your patch to hold the slab_mutex
> while calling this function, or to update the comment on top of this function
> saying that we are OK to call this function (even without slab_mutex) when we
> are inside a get/put_online_cpus() section.
>
> > if (s->flags & SLAB_DESTROY_BY_RCU)
> > @@ -179,8 +180,9 @@ void kmem_cache_destroy(struct kmem_cache *s)
> > s->name);
> > dump_stack();
>
> There is a list_add() before this dump_stack(). I assume we need to hold the
> slab_mutex while calling it.
Gah, of course it is, thanks for spotting this.
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_free()
call any more
- it silences the lockdep false positive warning, as it avoids lockdep ever
learning about slab_mutex -> cpu_hotplug.lock dependency
Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
---
mm/slab_common.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 9c21725..069a24e6 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -168,6 +168,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
list_del(&s->list);
if (!__kmem_cache_shutdown(s)) {
+ mutex_unlock(&slab_mutex);
if (s->flags & SLAB_DESTROY_BY_RCU)
rcu_barrier();
@@ -175,12 +176,14 @@ void kmem_cache_destroy(struct kmem_cache *s)
kmem_cache_free(kmem_cache, s);
} else {
list_add(&s->list, &slab_caches);
+ mutex_unlock(&slab_mutex);
printk(KERN_ERR "kmem_cache_destroy %s: Slab cache still has objects\n",
s->name);
dump_stack();
}
+ } else {
+ mutex_unlock(&slab_mutex);
}
- mutex_unlock(&slab_mutex);
put_online_cpus();
}
EXPORT_SYMBOL(kmem_cache_destroy);
--
Jiri Kosina
SUSE Labs
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