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

On Tue, Jan 26, 2016 at 02:33:40PM -0800, Linus Torvalds wrote:
> On Tue, Jan 26, 2016 at 2:15 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > 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.
> Just to clarify: on alpha it adds a memory barrier, but that memory
> barrier is useless.

No trailing data-dependent read, so agreed, no smp_read_barrier_depends()
needed.  That said, I believe that we should encourage rcu_dereference*()
or lockless_dereference() instead of READ_ONCE() for documentation
reasons, though.

> On non-alpha, it is a no-op, and obviously does nothing simply because
> it generates no code.
> So if anybody believes that the "smp_read_barrier_depends()" does
> something, they are *wrong*.

The other problem with smp_read_barrier_depends() is that it is often
a pain figuring out which prior load it is supposed to apply to.
Hence my preference for rcu_dereference*() and lockless_dereference().

> And if anybody sends out an email with that smp_read_barrier_depends()
> in an example, they are actively just confusing other people, which is
> even worse than just being wrong. Which is why I jumped in.
> So stop perpetuating the myth that smp_read_barrier_depends() does
> something here. It does not. It's a bug, and it has become this "mind
> virus" for some people that seem to believe that it does something.

It looks like I should add words to memory-barriers.txt de-emphasizing
smp_read_barrier_depends().  I will take a look at that.

> I had to remove this crap once from the kernel already, see commit
> 105ff3cbf225 ("atomic: remove all traces of READ_ONCE_CTRL() and
> atomic*_read_ctrl()").
> I don't want to ever see that broken construct again. And I want to
> make sure that everybody is educated about how broken it was. I'm
> extremely unhappy that it came up again.

Well, if it makes you feel better, that was control dependencies and this
was data dependencies.  So it was not -exactly- the same.  ;-)

(Sorry, couldn't resist...)

> If it turns out that some architecture does actually need a barrier
> between a read and a dependent write, then that will mean that
>  (a) we'll have to make up a _new_ barrier, because
> "smp_read_barrier_depends()" is not that barrier. We'll presumably
> then have to make that new barrier part of "rcu_derefence()" and
> friends.

Agreed.  We can worry about whether or not we replace the current
smp_read_barrier_depends() with that new barrier when and if such
hardware appears.

>  (b) we will have found an architecture with even worse memory
> ordering semantics than alpha, and we'll have to stop castigating
> alpha for being the worst memory ordering ever.

;-) ;-) ;-)

> but I sincerely hope that we'll never find that kind of broken architecture.

Apparently at least some hardware vendors are reading memory-barriers.txt,
so perhaps the odds of that kind of breakage have reduced.

                                                                Thanx, Paul

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