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

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Message-ID: <20140118100105.GV10038@linux.vnet.ibm.com>
Date:	Sat, 18 Jan 2014 02:01:05 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matt Turner <mattst88@...il.com>,
	Waiman Long <waiman.long@...com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Daniel J Blueman <daniel@...ascale.com>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

On Thu, Jan 16, 2014 at 11:36:59AM +0100, Peter Zijlstra wrote:
> On Thu, Jan 16, 2014 at 06:39:23AM +0700, Linus Torvalds wrote:
> > On Jan 16, 2014 6:22 AM, "Peter Zijlstra" <peterz@...radead.org> wrote:
> > >
> > > So while the primitive is called smp_store_release() the !SMP variant
> > > still does:
> > >
> > >   *(volatile __type *) = ptr;
> > >
> > > which should not compile on any Alpha pre EV56, SMP or no for __type ==
> > > u8.
> > 
> > I'm not sure where you get that "should not compile" theory from.
> > 
> > I'm pretty sure it will compile just fine. It will just generate the same
> > standard read-modify-write sequence (and not using the ldl/stc sequence
> > either). Do you have any actual reason to believe it won't, apart from your
> > theoretical wishes of how the world should work?
> 
> No, I earlier even said it probably would compile. My usage of 'should'
> comes from how we've 'defined' volatile/ACCESS_ONCE in
> Documentation/memory-barriers.txt. According to those constraints the
> rmw cycle is not proper code.

OK, I will bite...  Aside from fine-grained code timing, what code could
you write to tell the difference between a real one-byte store and an
RMW emulating that store?

							Thanx, Paul

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