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SubjectRe: [PATCH] selinux: reduce locking overhead in inode_free_security()
On 06/11/2015 08:38 AM, Stephen Smalley wrote:
> On 06/10/2015 04:17 PM, 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. We still need
>> to do the empty list test inside the lock for safety reason, but it
>> minimizes the chance of unnecessary spinlock contention.
>>
>> Signed-off-by: Waiman Long<Waiman.Long@hp.com>
>> ---
>> security/selinux/hooks.c | 17 +++++++++++++----
>> 1 files changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>> index 7dade28..cd736c3 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))
>> - list_del_init(&isec->list);
>> - spin_unlock(&sbsec->isec_lock);
>> + /*
>> + * 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. Lock taking can be slow
>> + * especially if the lock is being contended. We do, however, need
>> + * to recheck the list again before deleting it for safety.
>> + */
>> + if (!list_empty(&isec->list)) {
>> + spin_lock(&sbsec->isec_lock);
>> + if (!list_empty(&isec->list))
>> + list_del_init(&isec->list);
>> + spin_unlock(&sbsec->isec_lock);
>> + }
>>
>> /*
>> * The inode may still be referenced in a path walk and
>>
> Do we really need the second list_empty() test at all?
> Once removed, inode security structures are never re-added to the list.
> For comparison, inode_sb_list_del() only tests list_empty() outside the
> lock.
>

Yes, I think we can remove the second list_empty() test. I will update
the patch to do that.

Cheers,
Longman


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