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

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Message-ID: <20160126201339.GW4503@linux.vnet.ibm.com>
Date:	Tue, 26 Jan 2016 12:13:39 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Will Deacon <will.deacon@....com>,
	Leonid Yegoshin <Leonid.Yegoshin@...tec.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 Tue, Jan 26, 2016 at 11:19:27AM +0100, Peter Zijlstra wrote:
> On Mon, Jan 25, 2016 at 10:03:22PM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 25, 2016 at 04:42:43PM +0000, Will Deacon wrote:
> > > On Fri, Jan 15, 2016 at 01:58:53PM -0800, Paul E. McKenney wrote:
> > > > On Fri, Jan 15, 2016 at 10:27:14PM +0100, Peter Zijlstra wrote:
> 
> > > > > Yes, that seems a good start. But yesterday you raised the 'fun' point
> > > > > of two globally ordered sequences connected by a single local link.
> > > > 
> > > > The conclusion that I am slowly coming to is that litmus tests should
> > > > not be thought of as linear chains, but rather as cycles.  If you think
> > > > of it as a cycle, then it doesn't matter where the local link is, just
> > > > how many of them and how they are connected.
> > > 
> > > Do you have some examples of this? I'm struggling to make it work in my
> > > mind, or are you talking specifically in the context of the kernel
> > > memory model?
> > 
> > Now that you mention it, maybe it would be best to keep the transitive
> > and non-transitive separate for the time being anyway.  Just because it
> > might be possible to deal with does not necessarily mean that we should
> > be encouraging it.  ;-)
> 
> So isn't smp_mb__after_unlock_lock() exactly such a scenario? And would
> not someone trying to implement RCsc locks using locally transitive
> RELEASE/ACQUIRE operations need exactly this stuff?
> 
> That is, I am afraid we need to cover the mix of local and global
> transitive operations at least in overview.

True, but we haven't gotten to locking yet.  That said, I would argue
that smp_mb__after_unlock_lock() upgrades locks to transitive, and
thus would not be an exception to the "no combining transitive and
non-transitive steps in cycles" rule.

							Thanx, Paul