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

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Message-ID: <20160127005730.GE4503@linux.vnet.ibm.com>
Date:	Tue, 26 Jan 2016 16:57:30 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	linux-mips <linux-mips@...ux-mips.org>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Will Deacon <will.deacon@....com>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Peter Anvin <hpa@...or.com>, sparclinux@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	uml-devel <user-mode-linux-devel@...ts.sourceforge.net>,
	linux-sh@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
	the arch/x86 maintainers <x86@...nel.org>,
	xen-devel@...ts.xenproject.org, Ingo Molnar <mingo@...e.hu>,
	linux-xtensa@...ux-xtensa.org,
	James Hogan <james.hogan@...tec.com>,
	Arnd Bergmann <arnd@...db.de>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	adi-buildroot-devel@...ts.sourceforge.net,
	David Daney <ddaney.cavm@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-metag@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Joe Perches <joe@...ches.com>,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

On Tue, Jan 26, 2016 at 03:45:23PM -0800, Linus Torvalds wrote:
> On Tue, Jan 26, 2016 at 3:29 PM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> >
> > No trailing data-dependent read, so agreed, no smp_read_barrier_depends()
> > needed.  That said, I believe that we should encourage rcu_dereference*()
> > or lockless_dereference() instead of READ_ONCE() for documentation
> > reasons, though.
> 
> I agree that that is likely the right thing to do in pretty much all situations.
> 
> In theory, there might be performance situations where we'd want to
> actively avoid the smp_read_barrier_depends() inherent in those, but
> considering that it's only a performance issue on alpha, and we
> probably have all of two or three people using Linux on alpha, it's a
> pretty theoretical performance worry.

Agreed!

							Thanx, Paul