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

[Waiman Long]

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@xxxxxx>
> > ---
> >   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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