linux-kernel - Re: [RFC 10/12] x86, rwsem: simplify __down

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Message-ID: <20160609173640.GX30154@twins.programming.kicks-ass.net>
Date:	Thu, 9 Jun 2016 19:36:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Howells <dhowells@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Michal Hocko <mhocko@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	Tony Luck <tony.luck@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>, x86@...nel.org,
	linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	linux-arch@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Jason Low <jason.low2@....com>
Subject: Re: [RFC 10/12] x86, rwsem: simplify __down_write

On Thu, Jun 09, 2016 at 03:40:58PM +0100, David Howells wrote:
> Peter Zijlstra <peterz@...radead.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).