Re: [RFC 10/12] x86, rwsem: simplify __down_write
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- To: David Howells <dhowells@xxxxxxxxxx>
- Subject: Re: [RFC 10/12] x86, rwsem: simplify __down_write
- From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
- Date: Thu, 9 Jun 2016 19:36:40 +0200
- Cc: Ingo Molnar <mingo@xxxxxxxxxx>, Michal Hocko <mhocko@xxxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, "David S. Miller" <davem@xxxxxxxxxxxxx>, Tony Luck <tony.luck@xxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Chris Zankel <chris@xxxxxxxxxx>, Max Filippov <jcmvbkbc@xxxxxxxxx>, x86@xxxxxxxxxx, linux-alpha@xxxxxxxxxxxxxxx, linux-ia64@xxxxxxxxxxxxxxx, linux-s390@xxxxxxxxxxxxxxx, linux-sh@xxxxxxxxxxxxxxx, sparclinux@xxxxxxxxxxxxxxx, linux-xtensa@xxxxxxxxxxxxxxxx, linux-arch@xxxxxxxxxxxxxxx, Michal Hocko <mhocko@xxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, "Paul E. McKenney" <paulmck@xxxxxxxxxx>, Jason Low <jason.low2@xxxxxxx>
- In-reply-to: <29355.1465483258@warthog.procyon.org.uk>
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- References: <20160603223417.GB3892@twins.programming.kicks-ass.net> <1454444369-2146-1-git-send-email-mhocko@kernel.org> <1454444369-2146-11-git-send-email-mhocko@kernel.org> <20160203081016.GD32652@gmail.com> <20160603161339.GC3693@twins.programming.kicks-ass.net> <29355.1465483258@warthog.procyon.org.uk>
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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).
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