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

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Date:	Thu, 16 Jan 2014 11:36:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Matt Turner <mattst88@...il.com>, Waiman Long <waiman.long@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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.
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