linux-kernel - Re: [v3,11/41] mips: reuse asm-generic/barrier.h

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Date:	Tue, 26 Jan 2016 11:51:04 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Leonid.Yegoshin@...tec.com, linux-mips@...ux-mips.org,
	linux-ia64@...r.kernel.org, mst@...hat.com, peterz@...radead.org,
	will.deacon@....com, virtualization@...ts.linux-foundation.org,
	hpa@...or.com, sparclinux@...r.kernel.org, mingo@...nel.org,
	linux-arch@...r.kernel.org, linux-s390@...r.kernel.org,
	linux@....linux.org.uk,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-sh@...r.kernel.org, mpe@...erman.id.au, x86@...nel.org,
	xen-devel@...ts.xenproject.org, mingo@...e.hu,
	linux-xtensa@...ux-xtensa.org, james.hogan@...tec.com,
	arnd@...db.de, stefano.stabellini@...citrix.com,
	adi-buildroot-devel@...ts.sourceforge.net, ddaney.cavm@...il.com,
	tglx@...utronix.de, linux-metag@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, andrew.cooper3@...rix.com,
	linux-kernel@...r.kernel.org, ralf@...ux-mips.org, joe@...ches.com,
	linuxppc-dev@...ts.ozlabs.org, davem@...emloft.net,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

On Wed, Jan 27, 2016 at 12:52:07AM +0800, Boqun Feng wrote:
> Hi Paul,
> 
> On Mon, Jan 18, 2016 at 07:46:29AM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 18, 2016 at 04:19:29PM +0800, Herbert Xu wrote:
> > > Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > > >
> > > > You could use SYNC_ACQUIRE() to implement read_barrier_depends() and
> > > > smp_read_barrier_depends(), but SYNC_RMB probably does not suffice.
> > > > The reason for this is that smp_read_barrier_depends() must order the
> > > > pointer load against any subsequent read or write through a dereference
> > > > of that pointer.  For example:
> > > > 
> > > >        p = READ_ONCE(gp);
> > > >        smp_rmb();
> > > >        r1 = p->a; /* ordered by smp_rmb(). */
> > > >        p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */
> > > >        r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */
> > > > 
> > > > In contrast:
> > > > 
> > > >        p = READ_ONCE(gp);
> > > >        smp_read_barrier_depends();
> > > >        r1 = p->a; /* ordered by smp_read_barrier_depends(). */
> > > >        p->b = 42; /* ordered by smp_read_barrier_depends(). */
> > > >        r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */
> > > > 
> > > > Again, if your hardware maintains local ordering for address
> > > > and data dependencies, you can have read_barrier_depends() and
> > > > smp_read_barrier_depends() be no-ops like they are for most
> > > > architectures.
> > > > 
> > > > Does that help?
> > > 
> > > This is crazy! smp_rmb started out being strictly stronger than
> > > smp_read_barrier_depends, when did this stop being the case?
> > 
> > Hello, Herbert!
> > 
> > It is true that most Linux kernel code relies only on the read-read
> > properties of dependencies, but the read-write properties are useful.
> > Admittedly relatively rarely, but useful.
> > 
> > The better comparison for smp_read_barrier_depends(), especially in
> > its rcu_dereference*() form, is smp_load_acquire().
> 
> Confused..
> 
> I recall that last time you and Linus came into a conclusion that even
> on Alpha, a barrier for read->write with data dependency is unnecessary:
> 
> http://article.gmane.org/gmane.linux.kernel/2077661
> 
> And in an earlier mail of that thread, Linus made his point that
> smp_read_barrier_depends() should only be used to order read->read.

Those examples involved read-to-write with conditionals, as in:

	if (READ_ONCE(a))
		WRITE_ONCE(b, 1);

Without the "if", no ordering is guaranteed on weakly ordered CPUs.
(The volatile accesses keep ordering within the compiler for once...

> So right now, are we going to extend the semantics of
> smp_read_barrier_depends()? Can we just make smp_read_barrier_depends()
> still only work for read->read, and assume all the architectures won't
> reorder read->write with data dependency, so that the code above having
> a smp_rmb() also works?

The semantics of smp_read_barrier_depends() has been both read-to-write
and read-to-read for some time now, this patch just catches the
documentation up with reality.

							Thanx, Paul