Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc)

Paul E. McKenney
2014-06-10T12:52:43Z

On Tue, Jun 10, 2014 at 10:37:26AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 09, 2014 at 09:26:13AM -0700, Paul E. McKenney wrote:
> > That would indeed be a bad thing, as it could potentially lead to
> > use-after-free bugs. Though one could argue that any code that resulted
> > in use-after-free would be quite aggressive. But still...
>
> Let me hijack this thread for yet another issue... So I had an RCU
> related use-after-free the other day, and while Sasha was able to
> trigger it quite easily, I had a multi-day struggle to reproduce.
>
> Once I figured out what the exact problem was it was also clear to me
> why it was so hard for me to reproduce.
>
> So normally its easier to trigger races on bigger machines, more cpus,
> more concurrency, more races, all good.
>
> _However_ with RCU the grace period machinery is slower the bigger the
> machine, so bigger machine, slower grace period, slower RCU free, less
> likely to hit use-after-free.
>
> So I was thinking, and I know you all will go kick me for this because
> the very last thing we need is what I'm about to propose: more RCU
> flavours :-).
>
> How about an rcu_read_unlock() reference counted RCU variant that's
> ultra aggressive in doing the callbacks in order to better trigger such
> issues?

If you are using synchronize_rcu() for the update side, then I suggest
rcutorture.gp_exp=1 to force use expediting throughout.

Thanx, Paul