Re: [PATCH v2] selinux: reduce locking overhead in inode_free_security()

[Yury <yury.norov@gmail.com>]

On 13.06.2015 01:35, 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@hp.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.
>
> Cheers,
> Longman
>

Hello, Waiman!

At first, minor.
For me, moving the line 'if (!list_empty(&isec->list))' out of lock is 
not possible just because 'inode_free_security' is called from 
'__destroy_inode' only. You cannot rely on it in future. It's rather 
possible because empty list is invariant under 'list_del_init', as you 
noted here. In fact, you can call 'list_del_init' unconditionally here, 
and condition is the only optimization to decrease lock contention. So, 
I'd like to ask you reflect it in your comment.

At second, less minor.
Now that you access list element outside of the lock, why don't you use 
'list_empty_careful' instead of 'list_empty'? It may eliminate possible 
race between, say, 'list_add' and 'list_empty', and costs you virtually 
nothing.

Best regards,
Yury