Re: [PATCH] kernel: fix data race in put_pid

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

On Fri, Sep 18, 2015 at 10:28:20AM +0100, Will Deacon wrote:
> On Fri, Sep 18, 2015 at 10:06:46AM +0100, Dmitry Vyukov wrote:
> > On Fri, Sep 18, 2015 at 10:51 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > > As an alternative patch, could we not do:
> > >
> > >   void put_pid(struct pid *pid)
> > >   {
> > >         struct pid_namespace *ns;
> > >
> > >         if (!pid)
> > >                 return;
> > >
> > >         ns = pid->numbers[pid->level].ns;
> > >         if ((atomic_read(&pid->count) == 1) ||
> > >              atomic_dec_and_test(&pid->count)) {
> > >
> > > +               smp_read_barrier_depends(); /* ctrl-dep */
> > >
> > >                 kmem_cache_free(ns->pid_cachep, pid);
> > >                 put_pid_ns(ns);
> > >         }
> > >   }
> > >
> > > That would upgrade the atomic_read() path to a full READ_ONCE_CTRL(),
> > > and thereby avoid any of the kmem_cache_free() stores from leaking out.
> > > And its free, except on Alpha. Whereas the atomic_read_acquire() will
> > > generate a full memory barrier on whole bunch of archs.
> > 
> > 
> > What you propose makes sense.
> > 
> > +Will, Paul
> > 
> > Can we have something along the lines of:
> > 
> > #define atomic_read_ctrl(v) READ_ONCE_CTRL(&(v)->counter)
> 
> Funnily enough, I had this exact same discussion off-list yesterday
> afternoon, since I wrote some code relying on a ctrl dependency from
> an atomic_read to an atomic_xchg_relaxed.
> 
> So I guess I'm for the addition, but at the same time, could we make
> atomic_read and atomic_set generic too?

If we have some places that could use it, by all means let's add it!

							Thanx, Paul