On Wed, Dec 08, 2021 at 10:00:04AM -0800, Linus Torvalds wrote: > On Wed, Dec 8, 2021 at 9:07 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > IOW, the effective range becomes: [1..INT_MIN], which is a bit > > counter-intuitive, but then so is most of this stuff. > > I'd suggest not codifying it too strictly, because the exact range at > the upper end might depend on what is convenient for an architecture > to do. > > For x86, 'xadd' has odd semantics in that the flags register is about > the *new* state, but the returned value is about the *old* state. >From testing xadd had different flags from add; I've not yet looked at the SDM to see what it said on the matter. > That means that on x86, some things are cheaper to test based on the > pre-inc/dec values, and other things are cheaper to test based on the > post-inc/dec ones. > > It's also why for "page->_mapcount" we have the "free" value being -1, > not 0, and the refcount is "off by one". It makes the special cases of > "increment from zero" and "decrement to zero" be very easy and > straightforward to test for. > > That might be an option for an "atomic_ref" type - with our existing > "page_mapcount()" code being the thing we'd convert first, and make be > the example for it. > > I think it should also make the error cases be very easy to check for > without extra tests. If you make "decrement from zero" be the "ok, now > it's free", then that shows in the carry flag. But otherwise, if SF or > OF is set, it's an error. That means we can use the regular atomics > and flags (although not "dec" and "inc", since we'd care about CF). > > So on x86, I think "atomic_dec_ref()" could be > > lock subl $1,ptr > jc now_its_free > jl this_is_an_error > > if we end up having that "off by one" model. > > And importantly, "atomic_inc_ref()" would be just > > lock incl ptr > jle this_is_an_error > > and this avoids us having to have the value in a register and test it > separately. > > So your suggestion is _close_, but note how you can't do the "inc_ofl" > without that "off-by-one" model. > > And again - I might have gotten the exact flag test instructions > wrong. That's what you get for not actually doing serious assembly > language for a couple of decades. Yeah; I don't have it all in-cache either; I'll go through it tomorrow or something to see what I can make of it. Meanwhile I did send out what I had.
