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

[Eric Paris <eparis@redhat.com>]

On Fri, 2015-06-12 at 08:31 -0400, 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.

Wait, can't you list_del_init() an already list_del_init'd object.
Isn't that a big difference between list_del() and list_del_init() ?