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

["Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>]

On Wed, Jan 27, 2016 at 10:04:47AM +0800, Boqun Feng wrote:
> 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 a good start, but more is needed to warn people off of
smp_read_barrier_depends().  But yes, better explanation would be good.

                                                        Thanx, Paul

>       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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