Re: [RFC 10/12] x86, rwsem: simplify __down_write

[Peter Zijlstra]

On Thu, Jun 09, 2016 at 03:40:58PM +0100, David Howells wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > Blergh; so looking at more asm there's still a few tricks we cannot do.
> > So while overall size is down, some paths do end up more expensive. (It
> > typically boils down to creative use of condition flags, which is very
> > hard in C)
> 
> It can be done using ISO __atomic_fetch_add() and suchlike.

(ISO-C11, ISO as such is a bad abbreviation I think)

Maybe, but we're almost there with __GCC_ASM_FLAG_OUTPUTS__.

atomic_long_add_negative() can be made to do inc;j(n)s for __down_read.

the try_cmpxchg family you wanted to add independent from the ISO-C11
bits can do the cmpxchg-j(n)z for __down_{read,write}_trylock.

That only leaves us wanting an atomic_long_fetch_add_negative() for
__up_{read,write}().

Although I suppose, for this to be of use for our weakly ordered
friends, we need _relaxed versions of all that (so that _acquire and
_release variants are generated).