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

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

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.

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*.

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.

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.

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.

 (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.

               Linus

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