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

From	Linus Torvalds <torvalds@linux-foundation.org>
Newsgroups	linux.kernel
Subject	Re: [v3,11/41] mips: reuse asm-generic/barrier.h
Date	2016-01-26 23:20 +0100
Message-ID	<qVk7U-2la-9@gated-at.bofh.it> (permalink)
References	(2 earlier) <qSke6-2DG-11@gated-at.bofh.it> <qVf8f-6St-27@gated-at.bofh.it> <qVfBh-7mT-23@gated-at.bofh.it> <qVhMK-qV-7@gated-at.bofh.it> <qVjYf-2hg-31@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Tue, Jan 26, 2016 at 12:10 PM, Paul E. McKenney
<paulmck@linux.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

Thread

Re: [v3,11/41] mips: reuse asm-generic/barrier.h Herbert Xu <herbert@gondor.apana.org.au> - 2016-01-18 09:30 +0100
  Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-18 16:50 +0100
    Re: [v3,11/41] mips: reuse asm-generic/barrier.h Boqun Feng <boqun.feng@gmail.com> - 2016-01-26 18:00 +0100
      Re: [v3,11/41] mips: reuse asm-generic/barrier.h Peter Zijlstra <peterz@infradead.org> - 2016-01-26 18:30 +0100
        Re: [v3,11/41] mips: reuse asm-generic/barrier.h Linus Torvalds <torvalds@linux-foundation.org> - 2016-01-26 20:50 +0100
          Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-26 23:10 +0100
            Re: [v3,11/41] mips: reuse asm-generic/barrier.h Linus Torvalds <torvalds@linux-foundation.org> - 2016-01-26 23:20 +0100
              Re: [v3,11/41] mips: reuse asm-generic/barrier.h Linus Torvalds <torvalds@linux-foundation.org> - 2016-01-26 23:40 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-27 00:30 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h Linus Torvalds <torvalds@linux-foundation.org> - 2016-01-27 00:50 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-27 02:00 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h Boqun Feng <boqun.feng@gmail.com> - 2016-01-27 03:10 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h Peter Zijlstra <peterz@infradead.org> - 2016-01-27 09:00 +0100
      Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-26 23:10 +0100

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