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Date: Tue, 7 Jun 2016 14:00:16 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock_wait()
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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.

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.