On Mon, Jan 28, 2019 at 11:45:32PM +0100, Jann Horn wrote:
> On Mon, Jan 28, 2019 at 11:39 PM Paul E. McKenney <paul...@linux.ibm.com>
> wrote:
> > On Mon, Jan 28, 2019 at 05:07:07PM -0500, Mathieu Desnoyers wrote:
> > > Jann Horn identified a racy access to p->mm in the global expedited
> > > command of the membarrier system call.
> > >
> > > The suggested fix is to hold the task_lock() around the accesses to
> > > p->mm and to the mm_struct membarrier_state field to guarantee the
> > > existence of the mm_struct.
> > >
> > > Link:
> > > https://lore.kernel.org/lkml/cag48ez2g8ctf8dhs42tf37pthfr3y0rnooytmxvacm4u8yu...@mail.gmail.com
> > > Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
> [...]
> > > --- a/kernel/sched/membarrier.c
> > > +++ b/kernel/sched/membarrier.c
> > > @@ -81,12 +81,27 @@ static int membarrier_global_expedited(void)
> > >
> > > rcu_read_lock();
> > > p = task_rcu_dereference(&cpu_rq(cpu)->curr);
> > > - if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
> > > - MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
> > > - if (!fallback)
> > > - __cpumask_set_cpu(cpu, tmpmask);
> > > - else
> > > - smp_call_function_single(cpu, ipi_mb, NULL,
> > > 1);
> > > + /*
> > > + * Skip this CPU if the runqueue's current task is NULL or
> > > if
> > > + * it is a kernel thread.
> > > + */
> > > + if (p && READ_ONCE(p->mm)) {
> > > + bool mm_match;
> > > +
> > > + /*
> > > + * Read p->mm and access membarrier_state while
> > > holding
> > > + * the task lock to ensure existence of mm.
> > > + */
> > > + task_lock(p);
> > > + mm_match = p->mm &&
> > > (atomic_read(&p->mm->membarrier_state) &
> >
> > Are we guaranteed that this p->mm will be the same as the one loaded via
> > READ_ONCE() above?
>
> No; the way I read it, that's just an optimization and has no effect
> on correctness.
>
> > Either way, wouldn't it be better to READ_ONCE() it a
> > single time and use the same value everywhere?
>
> No; the first READ_ONCE() returns a pointer that you can't access
> because it wasn't read under a lock. You can only use it for a NULL
> check.
Ah, of course! Thank you both!
Thanx, Paul
