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

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Date:	Thu, 14 Jan 2016 15:33:40 -0800
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	<paulmck@...ux.vnet.ibm.com>
CC:	Will Deacon <will.deacon@....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 01/14/2016 02:55 PM, Paul E. McKenney wrote:
> OK, so it looks like Will was asking not about WRC+addr+addr, but instead
> about WRC+sync+addr.
(He actually asked twice about this and that too but skip this)

> 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.  This leads me to believe that your smp_mb() needs
> to use SYNC rather than SYNC_MB, as was the subject of earlier spirited
> discussion in this thread.

Don't be fooled here by words "ordered" and "completed" - it is HW 
design items and actually written poorly.
Just assume that SYNC_MB is absolutely the same as SYNC for any CPU and 
coherent device (besides performance). The difference can be in 
non-coherent devices because SYNC actually tries to make a barrier for 
them too. In some SoCs it is just the same because there is no need to 
barrier a non-coherent device (device register access usually strictly 
ordered... if there is no bridge in between).

>
> Suppose you have something like this:
> ...
> Does your hardware guarantee that it is not possible for all of r0,
> r1, r2, and r3 to be equal to zero at the end of the test, assuming
> that a, b, c, and d are all initially zero, and the four functions
> above run concurrently?

It is assumed to be so from Arch point of view. HW bugs are possible, of 
course.

> Another (more academic) case is this one, with x and y initially zero:
>
> ...
> Does SYNC_MB() prohibit r1 == 1 && r2 == 0 && r3 == 1 && r4 == 0?

It is assumed to be so from Arch point of view. HW bugs are possible, of 
course.

Note: I am not sure about ANY past MIPS R2 CPU because that stuff is 
implemented some time but nobody made it in Linux kernel (it was used by 
some vendor for non-Linux system). For that reason my patch for 
lightweight SYNCs has an option - implement it or implement a generic 
SYNC. It is possible that some vendor did it in different way but nobody 
knows or test it. But as a minimum - SYNC must be implemented in 
spinlocks/atomics/bitops, in recent P5600 it is proven that read can 
pass write in atomics.

MIPS R6 is a different story, I verified lightweight SYNCs from the 
beginning and it also should use SYNCs.

- Leonid.