Re: Is it OK to pass non-acquired objects to kfree?

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Sep 10, 2015 at 10:13:33AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 10, 2015 at 11:55:35AM +0200, Dmitry Vyukov wrote:
> > On Thu, Sep 10, 2015 at 1:31 AM, Christoph Lameter <cl@linux.com> wrote:
> > > On Wed, 9 Sep 2015, Paul E. McKenney wrote:

[ . . . ]

> > There are memory allocator implementations that do reads and writes of
> > the object, and there are memory allocator implementations that do not
> > do any barriers on fast paths. From this follows that objects must be
> > passed in quiescent state to kfree.
> > Now, kernel memory model says "A load-load control dependency requires
> > a full read memory barrier".
> > >From this follows that the following code is broken:
> > 
> > // kernel/pid.c
> >          if ((atomic_read(&pid->count) == 1) ||
> >               atomic_dec_and_test(&pid->count)) {
> >                  kmem_cache_free(ns->pid_cachep, pid);
> >                  put_pid_ns(ns);
> >          }
> > 
> > and it should be:
> > 
> > // kernel/pid.c
> >          if ((smp_load_acquire(&pid->count) == 1) ||
> 
> If Will Deacon's patch providing generic support for relaxed atomics
> made it in, we want:
> 
> 	  if ((atomic_read_acquire(&pid->count) == 1) ||
> 
> Otherwise, we need an explicit barrier.

And atomic_read_acquire() is in fact now in mainline, so it is the
best choice here.

							Thanx, Paul

> >               atomic_dec_and_test(&pid->count)) {
> >                  kmem_cache_free(ns->pid_cachep, pid);
> >                  put_pid_ns(ns);
> >          }
> > 
> > 
> > 
> > -- 
> > Dmitry Vyukov, Software Engineer, dvyukov@google.com
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