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

[Oleg Nesterov <oleg@redhat.com>]

On 06/11, Paul E. McKenney wrote:
>
> On Wed, Jun 11, 2014 at 07:59:34PM +0200, Oleg Nesterov wrote:
> > On 06/11, Paul E. McKenney wrote:
> > >
> > > I was thinking of ->boost_completion as the way to solve it easily, but
> > > what did you have in mind?
> >
> > I meant, rcu_boost() could probably just do "mtx->owner = t", we know that
> > it was unlocked by us and nobody else can use it until we set
> > t->rcu_boost_mutex.
>
> My concern with this is that rcu_read_unlock_special() could hypothetically
> get preempted (either by kernel or hypervisor), so that it might be a long
> time until it makes its reference.  But maybe that reference would be
> harmless in this case.

Confused... Not sure I understand what did you mean, and certainly I do not
understand how this connects to the proxy-locking method.

Could you explain?

> > And if we move it into rcu_node, then we can probably kill ->rcu_boost_mutex,
> > rcu_read_unlock_special() could check rnp->boost_mutex->owner == current.
>
> If this was anywhere near a hot code path, I would be sorely tempted.

Ah, but I didn't mean perfomance. I think it is always good to try to remove
something from task_struct, it is huge. I do not mean sizeof() in the first
place, the very fact that I can hardly understand the purpose of a half of its
members makes me sad ;)

Oleg.