linux-kernel - Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock

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Date:	Tue, 7 Jun 2016 20:45:53 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, manfred@...orfullife.com,
	dave@...olabs.net, paulmck@...ux.vnet.ibm.com, Waiman.Long@....com,
	tj@...nel.org, pablo@...filter.org, kaber@...sh.net,
	davem@...emloft.net, oleg@...hat.com,
	netfilter-devel@...r.kernel.org, sasha.levin@...cle.com,
	hofrat@...dl.org, jejb@...isc-linux.org, chris@...kel.net,
	rth@...ddle.net, dhowells@...hat.com, schwidefsky@...ibm.com,
	mpe@...erman.id.au, ralf@...ux-mips.org, linux@...linux.org.uk,
	rkuo@...eaurora.org, vgupta@...opsys.com, james.hogan@...tec.com,
	realmz6@...il.com, ysato@...rs.sourceforge.jp, tony.luck@...el.com,
	cmetcalf@...lanox.com
Subject: Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock_wait()

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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