linux-kernel - Re: [PATCH 3/3] mm/slub: Fix potential deadlock problem in slab_attr

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Date:   Tue, 11 Feb 2020 18:30:13 -0500
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/3] mm/slub: Fix potential deadlock problem in
 slab_attr_store()

On 2/10/20 6:10 PM, Andrew Morton wrote:
> On Mon, 10 Feb 2020 17:14:31 -0500 Waiman Long <longman@...hat.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