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

[Waiman Long <waiman.long@hp.com>]

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