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

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Wed, Jan 27, 2016 at 12:52:07AM +0800, Boqun Feng wrote:
> I recall that last time you and Linus came into a conclusion that even
> on Alpha, a barrier for read->write with data dependency is unnecessary:
> 
> http://article.gmane.org/gmane.linux.kernel/2077661
> 
> And in an earlier mail of that thread, Linus made his point that
> smp_read_barrier_depends() should only be used to order read->read.
> 
> So right now, are we going to extend the semantics of
> smp_read_barrier_depends()? Can we just make smp_read_barrier_depends()
> still only work for read->read, and assume all the architectures won't
> reorder read->write with data dependency, so that the code above having
> a smp_rmb() also works?

That discussions was about control dependencies. So writes that _depend_
on a prior read having an explicit value.

So something like:

        struct foo *x = READ_ONCE(*ptr);
        smp_read_barrier_depends()
        if (x->val == 5)
                x->bar = 5;

In that case, the load of x->val must be complete and its value
determined _before_ the store to x->bar can happen.

This is distinct from:

        struct foo *x = READ_ONCE(*ptr);
        smp_read_barrier_depends();
        x->bar = 5;

And its the second case where smp_read_barrier_depends() read->write
order matters.

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