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

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Date:	Tue, 26 Jan 2016 14:15:21 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	linux-mips <linux-mips@...ux-mips.org>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Will Deacon <will.deacon@....com>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Peter Anvin <hpa@...or.com>, sparclinux@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	uml-devel <user-mode-linux-devel@...ts.sourceforge.net>,
	linux-sh@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	xen-devel@...ts.xenproject.org, Ingo Molnar <mingo@...e.hu>,
	linux-xtensa@...ux-xtensa.org,
	James Hogan <james.hogan@...tec.com>,
	Arnd Bergmann <arnd@...db.de>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	adi-buildroot-devel@...ts.sourceforge.net,
	David Daney <ddaney.cavm@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-metag@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Joe Perches <joe@...ches.com>,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

On Tue, Jan 26, 2016 at 12:10 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Tue, Jan 26, 2016 at 11:44:46AM -0800, Linus Torvalds wrote:
>>
>> >         struct foo *x = READ_ONCE(*ptr);
>> >         smp_read_barrier_depends();
>> >         x->bar = 5;
>>
>> This case is complete BS. Stop perpetuating it. I already removed a
>> number of bogus cases of it, and I removed the incorrect documentation
>> that had this crap.
>
> If I understand your objection correctly, you want the above pattern
> expressed either like this:
>
>         struct foo *x = rcu_dereference(*ptr);
>         x->bar = 5;
>
> Or like this:
>
>         struct foo *x = lockless_dereference(*ptr);
>         x->bar = 5;
>
> Or am I missing your point?

You are entirely missing the point.

You might as well just write it as

    struct foo x = READ_ONCE(*ptr);
    x->bar = 5;

because that "smp_read_barrier_depends()" does NOTHING wrt the second write.

So what I am saying is simple: anybody who writes that
"smp_read_barrier_depends()" in there is just ttoally and completely
WRONG, and the fact that Peter wrote it out after I removed several
instances of that bloody f*cking idiocy is disturbing.

Don't do it. It's BS. It's wrong. Don't make excuses for it.

             Linus