> 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
