Re: [PATCH 3/3] mm/slub: Fix potential deadlock problem in slab_attr_store()

[Waiman Long <longman@redhat.com>]

On 2/10/20 6:10 PM, Andrew Morton wrote:
> On Mon, 10 Feb 2020 17:14:31 -0500 Waiman Long <longman@redhat.com> wrote:
>
>>>> --- a/mm/slub.c
>>>> +++ b/mm/slub.c
>>>> @@ -5536,7 +5536,12 @@ static ssize_t slab_attr_store(struct kobject *kobj,
>>>>  	if (slab_state >= FULL && err >= 0 && is_root_cache(s)) {
>>>>  		struct kmem_cache *c;
>>>>  
>>>> -		mutex_lock(&slab_mutex);
>>>> +		/*
>>>> +		 * Timeout after 100ms
>>>> +		 */
>>>> +		if (mutex_timed_lock(&slab_mutex, 100) < 0)
>>>> +			return -EBUSY;
>>>> +
>>> Oh dear.  Surely there's a better fix here.  Does slab really need to
>>> hold slab_mutex while creating that sysfs file?  Why?
>>>
>>> If the issue is two threads trying to create the same sysfs file
>>> (unlikely, given that both will need to have created the same cache)
>>> then can we add a new mutex specifically for this purpose?
>>>
>>> Or something else.
>>>
>> Well, the current code iterates all the memory cgroups to set the same
>> value in all of them. I believe the reason for holding the slab mutex is
>> to make sure that memcg hierarchy is stable during this iteration
>> process.
> But that is unrelated to creation of the sysfs file?
>
OK, I will take a closer look at that.

Cheers,
Longman