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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

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@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.
>

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?