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

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Date:	Fri, 15 Jan 2016 10:24:32 +0000
From:	Will Deacon <will.deacon@....com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"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 02:55:10PM -0800, Paul E. McKenney wrote:
> On Thu, Jan 14, 2016 at 01:36:50PM -0800, Leonid Yegoshin wrote:
> > On 01/14/2016 01:29 PM, Paul E. McKenney wrote:
> > >
> > >>On 01/14/2016 12:34 PM, Paul E. McKenney wrote:
> > >>>
> > >>>The WRC+addr+addr is OK because data dependencies are not required to be
> > >>>transitive, in other words, they are not required to flow from one CPU to
> > >>>another without the help of an explicit memory barrier.
> > >>I don't see any reliable way to fit WRC+addr+addr into "DATA
> > >>DEPENDENCY BARRIERS" section recommendation to have data dependency
> > >>barrier between read of a shared pointer/index and read the shared
> > >>data based on that pointer. If you have this two reads, it doesn't
> > >>matter the rest of scenario, you should put the dependency barrier
> > >>in code anyway. If you don't do it in WRC+addr+addr scenario then
> > >>after years it can be easily changed to different scenario which
> > >>fits some of scenario in "DATA DEPENDENCY BARRIERS" section and
> > >>fails.
> > >The trick is that lockless_dereference() contains an
> > >smp_read_barrier_depends():
> > >
> > >#define lockless_dereference(p) \
> > >({ \
> > >	typeof(p) _________p1 = READ_ONCE(p); \
> > >	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
> > >	(_________p1); \
> > >})
> > >
> > >Or am I missing your point?
> > 
> > WRC+addr+addr has no any barrier. lockless_dereference() has a
> > barrier. I don't see a common points between this and that in your
> > answer, sorry.
> 
> Me, I am wondering what WRC+addr+addr has to do with anything at all.

See my earlier reply [1] (but also, your WRC Linux example looks more
like a variant on WWC and I couldn't really follow it).

> <Going back through earlier email>
> 
> OK, so it looks like Will was asking not about WRC+addr+addr, but instead
> about WRC+sync+addr.  This would drop an smp_mb() into cpu2() in my
> earlier example, which needs to provide ordering.
> 
> I am guessing that the manual's "Older instructions which must be globally
> performed when the SYNC instruction completes" provides the equivalent
> of ARM/Power A-cumulativity, which can be thought of as transitivity
> backwards in time. 

I couldn't make that leap. In particular, the manual's "Detailed
Description" sections explicitly refer to program-order:

  Every synchronizable specified memory instruction (loads or stores or
  both) that occurs in the instruction stream before the SYNC
  instruction must reach a stage in the load/store datapath after which
  no instruction re-ordering is possible before any synchronizable
  specified memory instruction which occurs after the SYNC instruction
  in the instruction stream reaches the same stage in the load/store
  datapath.

Will

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/399765.html