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

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Date:	Thu, 14 Jan 2016 08:16:04 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....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 12:04:45PM +0000, Will Deacon wrote:
> On Wed, Jan 13, 2016 at 12:58:22PM -0800, Leonid Yegoshin wrote:
> > On 01/13/2016 12:48 PM, Peter Zijlstra wrote:
> > >On Wed, Jan 13, 2016 at 11:02:35AM -0800, Leonid Yegoshin wrote:
> > >
> > >>I ask HW team about it but I have a question - has it any relationship with
> > >>replacing MIPS SYNC with lightweight SYNCs (SYNC_WMB etc)?
> > >Of course. If you cannot explain the semantics of the primitives you
> > >introduce, how can we judge the patch.
> > >
> > >
> > You missed a point - it is a question about replacement of SYNC with
> > lightweight primitives. It is NOT a question about multithread system
> > behavior without any SYNC. The answer on a latest Will's question lies in
> > different area.
> 
> The reason we (Peter and I) care about this isn't because we enjoy being
> obstructive. It's because there is a whole load of core (i.e. portable)
> kernel code that is written to the *kernel* memory model. For example,
> the scheduler, RCU, mutex implementations, perf, drivers, you name it.
> 
> Consequently, it's important that the architecture back-ends implement
> these portable primitives (e.g. smp_mb()) in a way that satisfies the
> kernel memory model so that core code doesn't need to worry about the
> underlying architecture for synchronisation purposes. You could turn
> around and say "but if MIPS gets it wrong, then that's MIPS's problem",
> but actually not having a general understanding of the ordering guarantees
> provided by each architecture makes it very difficult for us to extend
> the kernel memory model in such a way that it can be implemented
> efficiently across the board *and* relied upon by core code.

What Will said!

Yes, you can cut corners within MIPS architecture-specific code,
but primitives that are used in the core kernel really do need to
work as expected.

							Thanx, Paul

> The virtio patch at the start of the thread doesn't particularly concern
> me. It's the other patches you linked to that implement acquire/release
> that have me worried.
> 
> Will
>