[PATCH 7/7] slub: drop mutex before deleting sysfs entry

[Glauber Costa]

Sasha Levin recently reported a lockdep problem resulting from the
new attribute propagation introduced by kmemcg series. In short,
slab_mutex will be called from within the sysfs attribute store
function. This will create a dependency, that will later be held
backwards when a cache is destroyed - since destruction occurs with
the slab_mutex held, and then calls in to the sysfs directory removal
function.

In this patch, I propose to adopt a strategy close to what
__kmem_cache_create does before calling sysfs_slab_add, and release the
lock before the call to sysfs_slab_remove. This is pretty much the last
operation in the kmem_cache_shutdown() path, so we could do better by
splitting this and moving this call alone to later on. This will fit
nicely when sysfs handling is consistent between all caches, but will
look weird now.

Lockdep info:

[  351.935003] ======================================================
[  351.937693] [ INFO: possible circular locking dependency detected ]
[  351.939720] 3.7.0-rc4-next-20121106-sasha-00008-g353b62f #117 Tainted: G        W
[  351.942444] -------------------------------------------------------
[  351.943528] trinity-child13/6961 is trying to acquire lock:
[  351.943528]  (s_active#43){++++.+}, at: [<ffffffff812f9e11>] sysfs_addrm_finish+0x31/0x60
[  351.943528]
[  351.943528] but task is already holding lock:
[  351.943528]  (slab_mutex){+.+.+.}, at: [<ffffffff81228a42>] kmem_cache_destroy+0x22/0xe0
[  351.943528]
[  351.943528] which lock already depends on the new lock.
[  351.943528]
[  351.943528]
[  351.943528] the existing dependency chain (in reverse order) is:
[  351.943528]
-> #1 (slab_mutex){+.+.+.}:
[  351.960334]        [<ffffffff8118536a>] lock_acquire+0x1aa/0x240
[  351.960334]        [<ffffffff83a944d9>] __mutex_lock_common+0x59/0x5a0
[  351.960334]        [<ffffffff83a94a5f>] mutex_lock_nested+0x3f/0x50
[  351.960334]        [<ffffffff81256a6e>] slab_attr_store+0xde/0x110
[  351.960334]        [<ffffffff812f820a>] sysfs_write_file+0xfa/0x150
[  351.960334]        [<ffffffff8127a220>] vfs_write+0xb0/0x180
[  351.960334]        [<ffffffff8127a540>] sys_pwrite64+0x60/0xb0
[  351.960334]        [<ffffffff83a99298>] tracesys+0xe1/0xe6
[  351.960334]
-> #0 (s_active#43){++++.+}:
[  351.960334]        [<ffffffff811825af>] __lock_acquire+0x14df/0x1ca0
[  351.960334]        [<ffffffff8118536a>] lock_acquire+0x1aa/0x240
[  351.960334]        [<ffffffff812f9272>] sysfs_deactivate+0x122/0x1a0
[  351.960334]        [<ffffffff812f9e11>] sysfs_addrm_finish+0x31/0x60
[  351.960334]        [<ffffffff812fa369>] sysfs_remove_dir+0x89/0xd0
[  351.960334]        [<ffffffff819e1d96>] kobject_del+0x16/0x40
[  351.960334]        [<ffffffff8125ed40>] __kmem_cache_shutdown+0x40/0x60
[  351.960334]        [<ffffffff81228a60>] kmem_cache_destroy+0x40/0xe0
[  351.960334]        [<ffffffff82b21058>] mon_text_release+0x78/0xe0
[  351.960334]        [<ffffffff8127b3b2>] __fput+0x122/0x2d0
[  351.960334]        [<ffffffff8127b569>] ____fput+0x9/0x10
[  351.960334]        [<ffffffff81131b4e>] task_work_run+0xbe/0x100
[  351.960334]        [<ffffffff81110742>] do_exit+0x432/0xbd0
[  351.960334]        [<ffffffff81110fa4>] do_group_exit+0x84/0xd0
[  351.960334]        [<ffffffff8112431d>] get_signal_to_deliver+0x81d/0x930
[  351.960334]        [<ffffffff8106d5aa>] do_signal+0x3a/0x950
[  351.960334]        [<ffffffff8106df1e>] do_notify_resume+0x3e/0x90
[  351.960334]        [<ffffffff83a993aa>] int_signal+0x12/0x17
[  351.960334]
[  351.960334] other info that might help us debug this:
[  351.960334]
[  351.960334]  Possible unsafe locking scenario:
[  351.960334]
[  351.960334]        CPU0                    CPU1
[  351.960334]        ----                    ----
[  351.960334]   lock(slab_mutex);
[  351.960334]                                lock(s_active#43);
[  351.960334]                                lock(slab_mutex);
[  351.960334]   lock(s_active#43);
[  351.960334]
[  351.960334]  *** DEADLOCK ***
[  351.960334]
[  351.960334] 2 locks held by trinity-child13/6961:
[  351.960334]  #0:  (mon_lock){+.+.+.}, at: [<ffffffff82b21005>] mon_text_release+0x25/0xe0
[  351.960334]  #1:  (slab_mutex){+.+.+.}, at: [<ffffffff81228a42>] kmem_cache_destroy+0x22/0xe0
[  351.960334]
[  351.960334] stack backtrace:
[  351.960334] Pid: 6961, comm: trinity-child13 Tainted: G        W    3.7.0-rc4-next-20121106-sasha-00008-g353b62f #117
[  351.960334] Call Trace:
[  351.960334]  [<ffffffff83a3c736>] print_circular_bug+0x1fb/0x20c
[  351.960334]  [<ffffffff811825af>] __lock_acquire+0x14df/0x1ca0
[  351.960334]  [<ffffffff81184045>] ? debug_check_no_locks_freed+0x185/0x1e0
[  351.960334]  [<ffffffff8118536a>] lock_acquire+0x1aa/0x240
[  351.960334]  [<ffffffff812f9e11>] ? sysfs_addrm_finish+0x31/0x60
[  351.960334]  [<ffffffff812f9272>] sysfs_deactivate+0x122/0x1a0
[  351.960334]  [<ffffffff812f9e11>] ? sysfs_addrm_finish+0x31/0x60
[  351.960334]  [<ffffffff812f9e11>] sysfs_addrm_finish+0x31/0x60
[  351.960334]  [<ffffffff812fa369>] sysfs_remove_dir+0x89/0xd0
[  351.960334]  [<ffffffff819e1d96>] kobject_del+0x16/0x40
[  351.960334]  [<ffffffff8125ed40>] __kmem_cache_shutdown+0x40/0x60
[  351.960334]  [<ffffffff81228a60>] kmem_cache_destroy+0x40/0xe0
[  351.960334]  [<ffffffff82b21058>] mon_text_release+0x78/0xe0
[  351.960334]  [<ffffffff8127b3b2>] __fput+0x122/0x2d0
[  351.960334]  [<ffffffff8127b569>] ____fput+0x9/0x10
[  351.960334]  [<ffffffff81131b4e>] task_work_run+0xbe/0x100
[  351.960334]  [<ffffffff81110742>] do_exit+0x432/0xbd0
[  351.960334]  [<ffffffff811243b9>] ? get_signal_to_deliver+0x8b9/0x930
[  351.960334]  [<ffffffff8117d402>] ? get_lock_stats+0x22/0x70
[  351.960334]  [<ffffffff8117d48e>] ? put_lock_stats.isra.16+0xe/0x40
[  351.960334]  [<ffffffff83a977fb>] ? _raw_spin_unlock_irq+0x2b/0x80
[  351.960334]  [<ffffffff81110fa4>] do_group_exit+0x84/0xd0
[  351.960334]  [<ffffffff8112431d>] get_signal_to_deliver+0x81d/0x930
[  351.960334]  [<ffffffff8117d48e>] ? put_lock_stats.isra.16+0xe/0x40
[  351.960334]  [<ffffffff8106d5aa>] do_signal+0x3a/0x950
[  351.960334]  [<ffffffff811c8b33>] ? rcu_cleanup_after_idle+0x23/0x170
[  351.960334]  [<ffffffff811cc1c4>] ? rcu_eqs_exit_common+0x64/0x3a0
[  351.960334]  [<ffffffff811caa5d>] ? rcu_user_enter+0x10d/0x140
[  351.960334]  [<ffffffff811cc8d5>] ? rcu_user_exit+0xc5/0xf0
[  351.960334]  [<ffffffff8106df1e>] do_notify_resume+0x3e/0x90
[  351.960334]  [<ffffffff83a993aa>] int_signal+0x12/0x17

Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
CC: Michal Hocko <mhocko@xxxxxxx>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
CC: Johannes Weiner <hannes@xxxxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
CC: Pekka Enberg <penberg@xxxxxxxxxx>
---
 mm/slub.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index fead2cd..0769ccc 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3197,8 +3197,19 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
 {
 	int rc = kmem_cache_close(s);
 
-	if (!rc)
+	if (!rc) {
+		/*
+		 * We do the same lock strategy around sysfs_slab_add, see
+		 * __kmem_cache_create. Because this is pretty much the last
+		 * operation we do and the lock will be released shortly after
+		 * that in slab_common.c, we could just move sysfs_slab_remove
+		 * to a later point in common code. We should do that when we
+		 * have a common sysfs framework for all allocators.
+		 */
+		mutex_unlock(&slab_mutex);
 		sysfs_slab_remove(s);
+		mutex_lock(&slab_mutex);
+	}
 
 	return rc;
 }
-- 
1.7.11.7

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