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

From: Will Deacon <will.deacon@arm.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-mips@linux-mips.org, linux-ia64@vger.kernel.org,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	virtualization@lists.linux-foundation.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	sparclinux@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
	linux-arch@vger.kernel.org, linux-s390@vger.kernel.org,
	Russell King - ARM Linux <linux@arm.linux.org.uk>,
	user-mode-linux-devel@lists.sourceforge.net,
	linux-sh@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>,
	x86@kernel.org, xen-devel@lists.xenproject.org,
	Ingo Molnar <mingo@elte.hu>,
	linux-xtensa@linux-xtensa.org, james.hogan@imgtec.com,
	Arnd Bergmann <arnd@arndb.de>,
	Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
	adi-buildroot-devel@lists.sourceforge.net,
	Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>,
	ddaney.cavm@gmail.com, Thomas Gleixner <tglx@linutronix.de>,
	linux-metag@vger.kernel.
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h
Date: Fri, 15 Jan 2016 10:24:32 +0000	[thread overview]
Message-ID: <20160115102431.GB2131__46360.9120496132$1452853549$gmane$org@arm.com> (raw)
In-Reply-To: <20160114225510.GJ3818@linux.vnet.ibm.com>

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