linux-kernel - Re: [PATCH v7 1/4] qrwlock: A queue read/write lock implementation

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Message-ID: <20131217194946.GO5919@linux.vnet.ibm.com>
Date:	Tue, 17 Dec 2013 11:49:46 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] qrwlock: A queue read/write lock implementation

On Tue, Dec 17, 2013 at 11:27:29AM -0800, Linus Torvalds wrote:
> On Tue, Dec 17, 2013 at 11:21 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> >
> > Looks like xadd() is x86-specific, but this is common code.  One
> > approach would be to do xadd() for other arches, another approach
> > would be to make .rw be an atomic_t rather than a u32.  Making it
> > be atomic_t is probably easiest.  (The cmpxchg()s would then need
> > to be atomic_cmpxchg().)
> 
> Note that "xadd()" has different semantics from "atomic_add_return()".

Gah, that one always trips me up.  :-/

> xadd() returns the original value, while atomic_add_return() returns
> the result of the addition.
> 
> In this case, we seem to want the xadd() semantics. I guess we can use
> "atomic_add_return(val,&atomic)-val" and just assume that the compiler
> gets it right (with the addition and the subtraction cancelling out).

That seems like it would work well.

> Or maybe we should have a "atomic_add_return_original()" with xadd
> semantics?

My lazy side prefers the autocancellation.  ;-)  But yes, there are a
number of architectures (including ARM and Power) where the compiler
would have to be very tricky to reach into an asm to do the cancellation.

So perhaps a generic atomic_add_return_original() that is defined in
terms of atomic_add_return() as you say above, allowing architectures
to override with more-efficient implementations?  The same could be done
for add_smp() and xadd(), for that matter.

							Thanx, Paul

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