linux-kernel - Re: [PATCH v2] selinux: reduce locking overhead in inode_free

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Date:	Mon, 15 Jun 2015 09:38:35 -0400
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>
CC:	James Morris <james.l.morris@...cle.com>,
	Scott J Norton <scott.norton@...com>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Douglas Hatch <doug.hatch@...com>, selinux@...ho.nsa.gov
Subject: Re: [PATCH v2] selinux: reduce locking overhead in inode_free_security()

On 06/14/2015 12:01 AM, Raghavendra K T wrote:
> On 06/13/2015 04:05 AM, Waiman Long wrote:
>> On 06/12/2015 08:31 AM, Stephen Smalley wrote:
>>> On 06/12/2015 02:26 AM, Raghavendra K T wrote:
>>>> On 06/12/2015 03:01 AM, Waiman Long wrote:
>>>>> The inode_free_security() function just took the superblock's
>>>>> isec_lock
>>>>> before checking and trying to remove the inode security struct from
>>>>> the
>>>>> linked list. In many cases, the list was empty and so the lock taking
>>>>> is wasteful as no useful work is done. On multi-socket systems with
>>>>> a large number of CPUs, there can also be a fair amount of spinlock
>>>>> contention on the isec_lock if many tasks are exiting at the same
>>>>> time.
>>>>>
>>>>> This patch changes the code to check the state of the list first
>>>>> before taking the lock and attempting to dequeue it. As this function
>>>>> is called indirectly from __destroy_inode(), there can't be another
>>>>> instance of inode_free_security() running on the same inode.
>>>>>
>>>>> Signed-off-by: Waiman Long<Waiman.Long@...com>
>>>>> ---
>>>>>    security/selinux/hooks.c |   15 ++++++++++++---
>>>>>    1 files changed, 12 insertions(+), 3 deletions(-)
>>>>>
>>>>> v1->v2:
>>>>>    - Take out the second list_empty() test inside the lock.
>>>>>
>>>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>>>>> index 7dade28..e5cdad7 100644
>>>>> --- a/security/selinux/hooks.c
>>>>> +++ b/security/selinux/hooks.c
>>>>> @@ -254,10 +254,19 @@ static void inode_free_security(struct inode
>>>>> *inode)
>>>>>        struct inode_security_struct *isec = inode->i_security;
>>>>>        struct superblock_security_struct *sbsec =
>>>>> inode->i_sb->s_security;
>>>>>
>>>>> -    spin_lock(&sbsec->isec_lock);
>>>>> -    if (!list_empty(&isec->list))
>>>>> +    /*
>>>>> +     * As not all inode security structures are in a list, we check
>>>>> for
>>>>> +     * empty list outside of the lock to make sure that we won't
>>>>> waste
>>>>> +     * time taking a lock doing nothing. As inode_free_security() is
>>>>> +     * being called indirectly from __destroy_inode(), there is no
>>>>> way
>>>>> +     * there can be two or more concurrent calls. So doing the
>>>>> list_empty()
>>>>> +     * test outside the loop should be safe.
>>>>> +     */
>>>>> +    if (!list_empty(&isec->list)) {
>>>>> +        spin_lock(&sbsec->isec_lock);
>>>>>            list_del_init(&isec->list);
>>>> Stupid question,
>>>>
>>>> I need to take a look at list_del_init() code, but it can so happen
>>>> that
>>>> if !list_empty() check could happen simultaneously, then serially two
>>>> list_del_init() can happen.
>>>>
>>>> is that not a problem()?
>>> Hmm...I suppose that's possible (sb_finish_set_opts and
>>> inode_free_security could both perform the list_del_init).  Ok, we'll
>>> stay with the first version.
>>>
>>
>> Actually, list_del_init() can be applied twice with no harm being done.
>> The first list_del_init() will set list-> next = list->prev = list. The
>> second one will do the same thing and so it should be safe.
>>
> 
> Waiman,
> I do not think it is just about list_del_init() twice
> 
> what if
> 
> 
> CPU1                   CPU2                         CPU3
> 
> !list_empty()         !list_empty()
> 
> lock
> list_del_init()
> unlock
> 
>                                                  list_add()
>                         lock
>                         list_del_init
>                         unlock
> 
> But this is valid only if list_add() is possible after first
> list_del_init. I need to see code though.
> OR am I missing something?

That should never be possible AFAICS.  So I guess the second version is
also safe.
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