Messages in this thread |  | | From | (Linus Torvalds) | Subject | Re: [Lse-tech] Re: RFC: patch to allow lock-free traversal of lists with insertion | Date | Wed, 10 Oct 2001 17:25:22 +0000 (UTC) |
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In article <20011010185848.D726@athlon.random>, Andrea Arcangeli <andrea@suse.de> wrote: > >However the more I think about it the more I suspect we'd better use >rmb() in all readers in the common code
Absolutely. It's not that expensive an operation on sane hardware. And it's definitely conceptually the only right thing to do - we're saying that we're doing a read that depends on a previous read having seen previous memory. Ergo, "rmb()".
Of course, right now Linux only exports a subset of the potential memory barriers, and maybe we should export a fuller set - allowing CPU's that have stricter ordering to possibly make it a no-op. But thinking about even something like x86, I don't see where Intel would guarantee that two reads (data-dependent or not) would have some implicit memory ordering.
Re-ordering reads with data dependencies is hard, but it happens quite naturally in a CPU that does address speculation. I don't know of anybody who does that, but I bet _somebody_ will. Maybe even the P4?
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