linux-kernel - Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write

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Message-ID: <20140115023958.GA10038@linux.vnet.ibm.com>
Date:	Tue, 14 Jan 2014 18:39:58 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Richard Henderson <rth@...ddle.net>,
	Matt Turner <mattst88@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Daniel J Blueman <daniel@...ascale.com>,
	Waiman Long <waiman.long@...com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>
Subject: Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

On Wed, Jan 15, 2014 at 07:25:04AM +0700, Linus Torvalds wrote:
> On Wed, Jan 15, 2014 at 6:44 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> >
> > Which means that Alpha should be able to similarly emulate 1-byte and
> > 2-byte atomics, correct?
> 
> Not reasonably, no.
> 
> The ldl/stc implementation on early alpha was so broken as to be
> unusable. It's not actually done in the cache, it WENT OUT ON THE BUS.
> We're talking 70's style "external lock signal" kind of things like
> the 8086 did for locked cycles before the advent of caches, the kind
> that nobody sane has done for a long long time.
> 
> So realistically, you absolutely do not want to use those things to
> emulate atomic byte/word accesses. The whole point of "load_acquire()"
> and "store_release()" is that it's supposed to be cheaper than a
> locked access, and can be done with just a barrier instruction or a
> special instruction flag.
> 
> If you just want to do a store release, on alpha you'd want to
> implement that as a full memory barrier followed by a store. It
> doesn't get the advantage of a real release consistency model, but at
> least it's not doing an external bus access. But you can only do that
> store as a 4-byte or 8-byte store.on the older alphas (byte and word
> stores work on newer ones).
> 
> Of course, it's entirely possible that nobody cares..

That would be my hope.  ;-)

If nobody cares about Alpha period, it is easy.  However, the last time
that I tried that approach, they sent me a URL of a wiki showing Alpha
systems still running mainline.  But a slow-but-working approach for
Alpha does seem reasonable, even for those still running Linux on Alpha.

							Thanx, Paul

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