On Tue, Nov 25, 2014 at 3:55 PM, Dennis Ferguson <dennis.c.fergu...@gmail.com> wrote: > > On 25 Nov, 2014, at 00:56 , Taylor R Campbell > <campbell+netbsd-tech-k...@mumble.net> wrote: >> My confusion came from that I thought memory ordering of load is more >> flexible in general. I also didn't quite understand "dependent-load". >> I have been only reading NetBSD kernel code to learn memory ordering; >> and I believe, nothing in the tree utilize this topologycal >> "dependent-load". I may be missing something though. >> >> We do have at least one use of nontrivial data-dependent loads >> in-tree, in pcq(9), as Dennis noted. > > More than that, I think. If I'm not mistaken there are also examples in > > sys/kern/kern_descrip.c > sys/kern/kern_tc.c > sys/kern/subr_ipi.c > sys/kern/vfs_cache.c > > When there is code which fills in a structure, followed by a > membar_producer(), > followed by an assignment to a pointer to point at the structure, it is very > unlikely that there will be barriers in the places where the pointer is > dereferenced.
I'd *really* appreciate if you leave comments explaining those (hidden) constraints.
