Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock_wait()

[Boqun Feng <boqun.feng@gmail.com>]

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On Tue, Jun 07, 2016 at 02:00:16PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 07, 2016 at 07:43:15PM +0800, Boqun Feng wrote:
> > On Mon, Jun 06, 2016 at 06:08:36PM +0200, Peter Zijlstra wrote:
> > > diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> > > index ce2f75e32ae1..e1c29d352e0e 100644
> > > --- a/kernel/locking/qspinlock.c
> > > +++ b/kernel/locking/qspinlock.c
> > > @@ -395,6 +395,8 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> > >  	 * pending stuff.
> > >  	 *
> > >  	 * p,*,* -> n,*,*
> > > +	 *
> > > +	 * RELEASE, such that the stores to @node must be complete.
> > >  	 */
> > >  	old = xchg_tail(lock, tail);
> > >  	next = NULL;
> > > @@ -405,6 +407,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> > >  	 */
> > >  	if (old & _Q_TAIL_MASK) {
> > >  		prev = decode_tail(old);
> > > +		/*
> > > +		 * The above xchg_tail() is also load of @lock which generates,
> > > +		 * through decode_tail(), a pointer.
> > > +		 *
> > > +		 * The address dependency matches the RELEASE of xchg_tail()
> > > +		 * such that the access to @prev must happen after.
> > > +		 */
> > > +		smp_read_barrier_depends();
> > 
> > Should this barrier be put before decode_tail()? Because it's the
> > dependency old -> prev that we want to protect here.
> 
> I don't think it matters one way or the other. The old->prev
> transformation is pure; it doesn't depend on any state other than old.
> 

But wouldn't the old -> prev transformation get broken on Alpha
semantically in theory? Because Alpha could reorder the LOAD part of the
xchg_tail() and decode_tail(), which results in prev points to other
cpu's mcs_node.

Though, this is fine in current code, because xchg_release() is actually
xchg() on Alpha, and Alpha doesn't use qspinlock.

> I put it between prev and dereferences of prev, because that's what made
> most sense to me; but really anywhere between the load of @old and the
> first dereference of @prev is fine I suspect.

I understand the barrier here serves more for a documentation purpose,
and I don't want to a paranoid ;-) I'm fine with the current place, just
thought we could put it at somewhere more conforming to our memory
model.

/me feel myself a paranoid anyway...

Regards,
Boqun

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