Re: [RFC PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

[Paul E. McKenney]

On Mon, Jan 28, 2019 at 12:27:03PM -0800, Linus Torvalds wrote:
> On Mon, Jan 28, 2019 at 10:27 AM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> 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.
> 
> Hmm. I think this is right. You shouldn't access another threads mm
> pointer without proper locking.
> 
> That said, we *could* make the mm_cachep be SLAB_TYPESAFE_BY_RCU,
> which would allow speculatively reading data off the mm pointer under
> RCU. It might not be the *right* mm if somebody just did an exit, but
> for things like this it shouldn't matter.

That sounds much simpler and more effective than the contention-reduction
approach that I suggested.  ;-)

							Thanx, Paul

> But if this is the only case that might care, it sounds like just
> doing the proper locking is the right approach.
> 
>            Linus
>