On Tue, Jan 26, 2016 at 03:29:21PM -0800, Paul E. McKenney wrote:
> 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().
>
Because semantically speaking, rcu_derefence*() and
lockless_dereference() are CONSUME(i.e. data/address dependent
read->read and read->write pairs are ordered), whereas
smp_read_barrier_depends() only guarantees read->read pairs with data
dependency are ordered, right?
If so, maybe we need to call it out in memory-barriers.txt, for example:
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 904ee42..6b262c2 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1703,8 +1703,8 @@ There are some more advanced barrier functions:
(*) lockless_dereference();
- This can be thought of as a pointer-fetch wrapper around the
- smp_read_barrier_depends() data-dependency barrier.
+ This is a load, and any load or store that has a data dependency on the
+ value returned by this load won't be reordered before this load.
This is also similar to rcu_dereference(), but in cases where
object lifetime is handled by some mechanism other than RCU, for
Regards,
Boqun
> > 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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