Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

Paul E. McKenney
2015-11-12T14:40:04Z

On Thu, Nov 12, 2015 at 04:00:58PM +0100, Oleg Nesterov wrote:
> On 11/12, Boqun Feng wrote:
> >
> > On Wed, Nov 11, 2015 at 08:39:53PM +0100, Oleg Nesterov wrote:
> > >
> > > object_t *object;
> > > spinlock_t lock;
> > >
> > > void update(void)
> > > {
> > > object_t *o;
> > >
> > > spin_lock(&lock);
> > > o = READ_ONCE(object);
> > > if (o) {
> > > BUG_ON(o->dead);
> > > do_something(o);
> > > }
> > > spin_unlock(&lock);
> > > }
> > >
> > > void destroy(void) // can be called only once, can't race with itself
> > > {
> > > object_t *o;
> > >
> > > o = object;
> > > object = NULL;
> > >
> > > /*
> > > * pairs with lock/ACQUIRE. The next update() must see
> > > * object == NULL after spin_lock();
> > > */
> > > smp_mb();
> > >
> > > spin_unlock_wait(&lock);
> > >
> > > /*
> > > * pairs with unlock/RELEASE. The previous update() has
> > > * already passed BUG_ON(o->dead).
> > > *
> > > * (Yes, yes, in this particular case it is not needed,
> > > * we can rely on the control dependency).
> > > */
> > > smp_mb();
> > >
> > > o->dead = true;
> > > }
> > >
> > > I believe the code above is correct and it needs the barriers on both sides.
> > >
> >
> > Hmm.. probably incorrect.. because the ACQUIRE semantics of spin_lock()
> > only guarantees that the memory operations following spin_lock() can't
> > be reorder before the *LOAD* part of spin_lock() not the *STORE* part,
> > i.e. the case below can happen(assuming the spin_lock() is implemented
> > as ll/sc loop)
> >
> > spin_lock(&lock):
> > r1 = *lock; // LL, r1 == 0
> > o = READ_ONCE(object); // could be reordered here.
> > *lock = 1; // SC
> >
> > This could happen because of the ACQUIRE semantics of spin_lock(), and
> > the current implementation of spin_lock() on PPC allows this happen.
> >
> > (Cc PPC maintainers for their opinions on this one)
>
> In this case the code above is obviously wrong. And I do not understand
> how we can rely on spin_unlock_wait() then.
>
> And afaics do_exit() is buggy too then, see below.
>
> > I think it's OK for it as an ACQUIRE(with a proper barrier) or even just
> > a control dependency to pair with spin_unlock(), for example, the
> > following snippet in do_exit() is OK, except the smp_mb() is redundant,
> > unless I'm missing something subtle:
> >
> > /*
> > * The setting of TASK_RUNNING by try_to_wake_up() may be delayed
> > * when the following two conditions become true.
> > * - There is race condition of mmap_sem (It is acquired by
> > * exit_mm()), and
> > * - SMI occurs before setting TASK_RUNINNG.
> > * (or hypervisor of virtual machine switches to other guest)
> > * As a result, we may become TASK_RUNNING after becoming TASK_DEAD
> > *
> > * To avoid it, we have to wait for releasing tsk->pi_lock which
> > * is held by try_to_wake_up()
> > */
> > smp_mb();
> > raw_spin_unlock_wait(&tsk->pi_lock);
>
> Perhaps it is me who missed something. But I don't think we can remove
> this mb(). And at the same time it can't help on PPC if I understand
> your explanation above correctly.

I cannot resist suggesting that any lock that interacts with
spin_unlock_wait() must have all relevant acquisitions followed by
smp_mb__after_unlock_lock().

Thanx, Paul