Subject: Re: Hung task when calling clone() due to netfilter/slab

From: Sasha Levin <[email-masked]>
Date: Sun, 15 Jan 2012 14:59:44 +0200

On Sat, 2012-01-14 at 18:10 +0100, Eric Dumazet wrote:
> Apparently SLUB calls sysfs_slab_add() from kmem_cache_create() while
> still holding slub_lock.
> 
> So if the task launched needs to "cat /proc/slabinfo" or anything
> needing slub_lock, its a deadlock.

I've made the following patch to test it, It doesn't look like it's the correct solution, but it verifies that the problem is there (it works well with the patch).

---------------

>>From cc4874b491b8e5d9d1ea5bf2032413efdbddced8 Mon Sep 17 00:00:00 2001
From: Sasha Levin <[email-masked]>
Date: Sun, 15 Jan 2012 14:55:03 +0200
Subject: [PATCH] slab: Fix hang when creating sysfs entries

This patch fixes the hang which happens when we create a sysfs entry and call
back to userspace. If the usermode helper tries to do anything which involves
slub_lock we will hang since slub_lock is already held.

Signed-off-by: Sasha Levin <[email-masked]>
---
 mm/slub.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 4907563..6948327 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5283,7 +5283,9 @@ static int sysfs_slab_add(struct kmem_cache *s)
 		kobject_put(&s->kobj);
 		return err;
 	}
+	up_write(&slub_lock);
 	kobject_uevent(&s->kobj, KOBJ_ADD);
+	down_write(&slub_lock);
 	if (!unmergeable) {
 		/* Setup first alias */
 		sysfs_slab_alias(s, s->name);
-- 
1.7.8.3

-- 

Sasha.