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

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Message-ID: <20160115085554.GF3421@worktop>
Date:	Fri, 15 Jan 2016 09:55:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	Will Deacon <will.deacon@....com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	virtualization@...ts.linux-foundation.org,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Joe Perches <joe@...ches.com>,
	David Miller <davem@...emloft.net>, linux-ia64@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-metag@...r.kernel.org, linux-mips@...ux-mips.org,
	x86@...nel.org, user-mode-linux-devel@...ts.sourceforge.net,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	xen-devel@...ts.xenproject.org, Ralf Baechle <ralf@...ux-mips.org>,
	Ingo Molnar <mingo@...nel.org>, ddaney.cavm@...il.com,
	james.hogan@...tec.com, Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

On Thu, Jan 14, 2016 at 01:29:13PM -0800, Paul E. McKenney wrote:
> So smp_mb() provides transitivity, as do pairs of smp_store_release()
> and smp_read_acquire(), 

But they provide different grades of transitivity, which is where all
the confusion lays.

smp_mb() is strongly/globally transitive, all CPUs will agree on the order.

Whereas the RCpc release+acquire is weakly so, only the two cpus
involved in the handover will agree on the order.