Re: [PATCH] block: switch to atomic_t for request references

Peter Zijlstra
2021-12-08T18:44:21Z

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@infradead.org> 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.