linux-kernel - Re: [PATCHv3] mm/slab: fix race with dereferencing NULL ptr in alloc_calls

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Date:	Wed, 3 Feb 2016 14:30:59 +0300
From:	Dmitry Safonov <dsafonov@...tuozzo.com>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
CC:	<akpm@...ux-foundation.org>, <cl@...ux.com>, <penberg@...nel.org>,
	<rientjes@...gle.com>, <iamjoonsoo.kim@....com>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<0x7f454c46@...il.com>
Subject: Re: [PATCHv3] mm/slab: fix race with dereferencing NULL ptr in
 alloc_calls_show

On 02/03/2016 12:44 PM, Vladimir Davydov wrote:
> On Wed, Feb 03, 2016 at 10:52:13AM +0300, Dmitry Safonov wrote:
> ...
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index b50aef0..2bfc0b1 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -451,6 +451,8 @@ EXPORT_SYMBOL(kmem_cache_create);
>>   static int shutdown_cache(struct kmem_cache *s,
>>   		struct list_head *release, bool *need_rcu_barrier)
>>   {
>> +	sysfs_slab_remove(s);
>> +
> shutdown_cache is called with slab_mutex held. slab_attr_store may take
> the mutex. So placing sysfs_slab_remove here introduces a potential
> deadlock.
>
>>   	if (__kmem_cache_shutdown(s) != 0)
>>   		return -EBUSY;
>>   
>> @@ -468,13 +470,8 @@ static void release_caches(struct list_head *release, bool need_rcu_barrier)
>>   	if (need_rcu_barrier)
>>   		rcu_barrier();
>>   
>> -	list_for_each_entry_safe(s, s2, release, list) {
>> -#ifdef SLAB_SUPPORTS_SYSFS
>> -		sysfs_slab_remove(s);
>> -#else
>> +	list_for_each_entry_safe(s, s2, release, list)
>>   		slab_kmem_cache_release(s);
>> -#endif
>> -	}
>>   }
>>   
>>   #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
>> @@ -614,6 +611,7 @@ void memcg_destroy_kmem_caches(struct mem_cgroup *memcg)
>>   	list_for_each_entry_safe(s, s2, &slab_caches, list) {
>>   		if (is_root_cache(s) || s->memcg_params.memcg != memcg)
>>   			continue;
>> +
> Please remove this hunk.
>
>>   		/*
>>   		 * The cgroup is about to be freed and therefore has no charges
>>   		 * left. Hence, all its caches must be empty by now.
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 2e1355a..b6a68b7 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -5296,11 +5296,6 @@ static void memcg_propagate_slab_attrs(struct kmem_cache *s)
>>   #endif
>>   }
>>   
>> -static void kmem_cache_release(struct kobject *k)
>> -{
>> -	slab_kmem_cache_release(to_slab(k));
>> -}
>> -
>>   static const struct sysfs_ops slab_sysfs_ops = {
>>   	.show = slab_attr_show,
>>   	.store = slab_attr_store,
>> @@ -5308,7 +5303,6 @@ static const struct sysfs_ops slab_sysfs_ops = {
>>   
>>   static struct kobj_type slab_ktype = {
>>   	.sysfs_ops = &slab_sysfs_ops,
>> -	.release = kmem_cache_release,
> I surmise this will resurrect the bug that was fixed by 41a212859a4dd
> ("slub: use sysfs'es release mechanism for kmem_cache").
So, I move __kmem_cache_shutdown into slab_kmem_cache_release,
release list will still be collected under slab_mutex.
Seems like no other way to do this.
Will resend mended version.
>
> Thanks,
> Vladimir


-- 
Regards,
Dmitry Safonov