Date: Wed, 30 Apr 2014 18:37:24 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <davidlohr@hp.com>
Cc: Ingo Molnar <mingo@kernel.org>, Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>, Alex Shi <alex.shi@linaro.org>,
        Andi Kleen <andi@firstfloor.org>,
        Michel Lespinasse <walken@google.com>, Rik van Riel <riel@redhat.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E.McKenney" <paulmck@linux.vnet.ibm.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        "Norton, Scott J" <scott.norton@hp.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] rwsem: Support optimistic spinning
Message-ID: <20140430163724.GH11096@twins.programming.kicks-ass.net>
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On Wed, Apr 30, 2014 at 09:32:17AM -0700, Davidlohr Bueso wrote:
> On Wed, 2014-04-30 at 11:21 +0200, Peter Zijlstra wrote:
> > On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote:
> > > +#ifdef CONFIG_SMP
> > 
> > > +#else
> > > +static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
> > > +{
> > > +	return false;
> > > +}
> > > +#endif
> > 
> > On the mutex side we guard this with MUTEX_SPIN_ON_OWNER, do we want to
> > use that here too?
> 
> I thought of it, but hated adding mutex naming to rwsem code -- we
> already do it for cpu_relax() thanks to s390.

Yah, we need to find a better name for that thing.

> MUTEX_SPIN_ON_OWNER depends on SMP && !DEBUG_MUTEXES.
> Right now rwsem optimistic spinning depends on SMP && RWSEM_XCHGADD_ALGORITHM.
> 
> It might sense to add RWSEM_SPIN_ON_OWNER to encapsulate what we already
> have. I don't think we want DEBUG_MUTEX dependencies in rwsems. Would
> you accept such thing?

Ah, I remember why we have that MUTEX_SPIN_ON_OWNER, its in the comment
in mutex.c right under using it. It the games we play with ->owner isn't
compatible with what DEBUG_MUTEXES wants it for.

So no, you're fine with the rwsem code as is.
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