Messages in this thread | | | Date | Thu, 9 Jun 2016 19:36:40 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC 10/12] x86, rwsem: simplify __down_write |
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On Thu, Jun 09, 2016 at 03:40:58PM +0100, David Howells wrote: > Peter Zijlstra <peterz@infradead.org> 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).
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