slab vs lockdep vs debugobjects

[Peter Zijlstra <peterz@infradead.org>]

Hi Pekka,

Thomas found a fun lockdep splat, see below. Basically call_rcu() can
end up in kmem_cache_alloc(), and call_rcu() is used under
l3->list_lock, causing the splat. Since the debug kmem_cache isn't 
SLAB_DESTROY_BY_RCU this shouldn't ever actually recurse.

Now, since this particular kmem_cache is created with
SLAB_DEBUG_OBJECTS, we thought it might be easy enough to set a separate
lockdep class for its l3->list_lock's.

However I found that the existing lockdep annotation is for kmalloc only
-- don't custom kmem_caches use OFF_SLAB?

Anyway, I got lost in slab (again), but would it make sense to move all
lockdep fixups into kmem_list3_init() or thereabouts?


---
=============================================
[ INFO: possible recursive locking detected ]
3.0.0-rc3+ #37
---------------------------------------------
udevd/124 is trying to acquire lock:
 (&(&parent->list_lock)->rlock){......}, at: [<ffffffff81119619>] ____cache_alloc+0xc9/0x323

but task is already holding lock:
 (&(&parent->list_lock)->rlock){......}, at: [<ffffffff8111844e>] __cache_free+0x325/0x3ea

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&(&parent->list_lock)->rlock);
  lock(&(&parent->list_lock)->rlock);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

2 locks held by udevd/124:
 #0:  (&(&(*({ do { const void *__vpp_verify = (typeof((&(slab_lock))))((void *)0); (void)__vpp_verify; } while (0); ({ unsigned long __ptr; __asm__ ("" : "=r"(__ptr) : "0"((typeof(*(&(slab_lock))) *)(&(slab_lock)))); (typeof((typeof(*(&(slab_lock))) *)(&(slab_lock)))) (__ptr + (((__per_cpu_offset[__cpu])))); }); })).lock)->rlock){..-...}, at: [<ffffffff811164cc>] __local_lock_irq+0x16/0x61
 #1:  (&(&parent->list_lock)->rlock){......}, at: [<ffffffff8111844e>] __cache_free+0x325/0x3ea

stack backtrace:
Pid: 124, comm: udevd Not tainted 3.0.0-rc3+ #37
Call Trace:
 [<ffffffff81081e3d>] __lock_acquire+0x9ae/0xdc8
 [<ffffffff8107f289>] ? look_up_lock_class+0x5f/0xbe
 [<ffffffff810812e4>] ? mark_lock+0x2d/0x1d8
 [<ffffffff81119619>] ? ____cache_alloc+0xc9/0x323
 [<ffffffff81082774>] lock_acquire+0x103/0x12e
 [<ffffffff81119619>] ? ____cache_alloc+0xc9/0x323
 [<ffffffff8107f6b9>] ? register_lock_class+0x1e/0x2ca
 [<ffffffff81247054>] ? __debug_object_init+0x43/0x2e7
 [<ffffffff814a7730>] _raw_spin_lock+0x3b/0x4a
 [<ffffffff81119619>] ? ____cache_alloc+0xc9/0x323
 [<ffffffff81119619>] ____cache_alloc+0xc9/0x323
 [<ffffffff8107f6b9>] ? register_lock_class+0x1e/0x2ca
 [<ffffffff81247054>] ? __debug_object_init+0x43/0x2e7
 [<ffffffff8111b0d5>] kmem_cache_alloc+0xc5/0x1fb
 [<ffffffff81247054>] __debug_object_init+0x43/0x2e7
 [<ffffffff8124735f>] ? debug_object_activate+0x38/0xdc
 [<ffffffff810812e4>] ? mark_lock+0x2d/0x1d8
 [<ffffffff8124730c>] debug_object_init+0x14/0x16
 [<ffffffff8106bd26>] rcuhead_fixup_activate+0x2b/0xbc
 [<ffffffff81246d6f>] debug_object_fixup+0x1e/0x2b
 [<ffffffff812473f6>] debug_object_activate+0xcf/0xdc
 [<ffffffff81118b93>] ? kmem_cache_shrink+0x68/0x68
 [<ffffffff810b1fc0>] __call_rcu+0x4f/0x19e
 [<ffffffff810b2124>] call_rcu+0x15/0x17
 [<ffffffff81117c4a>] slab_destroy+0x11f/0x157
 [<ffffffff81117dd4>] free_block+0x152/0x18d
 [<ffffffff81118497>] __cache_free+0x36e/0x3ea
 [<ffffffff81103b3b>] ? anon_vma_free+0x3d/0x41
 [<ffffffff811164cc>] ? __local_lock_irq+0x16/0x61
 [<ffffffff81117aad>] kmem_cache_free+0xa1/0x11f
 [<ffffffff81103b3b>] anon_vma_free+0x3d/0x41
 [<ffffffff81104a77>] __put_anon_vma+0x38/0x3d
 [<ffffffff81104aa5>] put_anon_vma+0x29/0x2d
 [<ffffffff81104b7e>] unlink_anon_vmas+0x72/0xa5
 [<ffffffff810faa5b>] free_pgtables+0x6c/0xcb
 [<ffffffff81100c96>] exit_mmap+0xc0/0xf7
 [<ffffffff8104de1d>] mmput+0x60/0xd3
 [<ffffffff81054112>] exit_mm+0x141/0x14e
 [<ffffffff814a7d75>] ? _raw_spin_unlock_irq+0x54/0x61
 [<ffffffff8105436a>] do_exit+0x24b/0x74f
 [<ffffffff811289ae>] ? fput+0x1d4/0x1e3
 [<ffffffff8107f539>] ? trace_hardirqs_off_caller+0x33/0x90
 [<ffffffff814a847d>] ? retint_swapgs+0x13/0x1b
 [<ffffffff81054ae2>] do_group_exit+0x82/0xad
 [<ffffffff81054b24>] sys_exit_group+0x17/0x1b
 [<ffffffff814ae182>] system_call_fastpath+0x16/0x1b