> From: Torvald Riegel
> > Oh, one of my favorite (NOT!) pieces of code in the kernel is the
> > implementation of the
> >
> > smp_read_barrier_depends()
> >
> > macro, which on every single architecture except for one (alpha) is a
> no-op.
> >
> > We have basically 30 or so empty definitions for it, and I think we
> > have something like five uses of it. One of them, I think, is
> > performance crticial, and the reason for that macro existing.
> >
> > What does it do? The semantics is that it's a read barrier between
> two
> > different reads that we want to happen in order wrt two writes on the
> > writing side (the writing side also has to have a "smp_wmb()" to
> order
> > those writes). But the reason it isn't a simple read barrier is that
> > the reads are actually causally *dependent*, ie we have code like
> >
> > first_read = read_pointer;
> > smp_read_barrier_depends();
> > second_read = *first_read;
> >
> > and it turns out that on pretty much all architectures (except for
> > alpha), the *data*dependency* will already guarantee that the CPU
> > reads the thing in order. And because a read barrier can actually be
> > quite expensive, we don't want to have a read barrier for this case.
>
> I don't have time to look at this in detail right now, but it looks
> roughly close to C++11's memory_order_consume to me, which is somehwat
> like an acquire, but just for subsequent data-dependent loads. Added
> for performance reasons on some architecture AFAIR.
>
It's intended to address the same problem, though somewhat differently. (I suspect there was overlap in the people involved?) One reason that C11 took a slightly different path is that compilers can, and sometimes do, remove dependencies, making smp_read_barrier_depends brittle unless it also imposes compiler constraints.
Hans
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