linux-kernel - Re: [PATCH 4/4] locking: Introduce smp_cond

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Date:	Thu, 12 Nov 2015 06:40:04 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Boqun Feng <boqun.feng@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, corbet@....net, mhocko@...nel.org,
	dhowells@...hat.com, torvalds@...ux-foundation.org,
	will.deacon@....com, Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

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

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