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

[Stephen Smalley]

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@xxxxxx>
>> ---
>>   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.

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