On Tue, Jan 26, 2016 at 02:33:40PM -0800, Linus Torvalds wrote:

On Tue, Jan 26, 2016 at 2:15 PM, Linus Torvalds
[off-list ref] wrote:

quoted

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