linux-kernel - Re: [PATCH RFC] locking: Add volatile to arch_spinlock

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Message-ID: <20141204220657.GB25340@linux.vnet.ibm.com>
Date:	Thu, 4 Dec 2014 14:06:57 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH RFC] locking: Add volatile to arch_spinlock_t structures

On Thu, Dec 04, 2014 at 09:45:46PM +0000, One Thousand Gnomes wrote:
> > anywhere in that translation unit.  After all, any non-static function
> > in that translation unit might be called from some other translation
> > unit that -did- use locking or whatever.
> > 
> > I will let you know how it goes.  ;-)
> 
> It breaks DEC10 ;-)

To say nothing of CDC 6600 systems lacking the compare-move unit.  ;-)

> If there is kickback over things like optimisation perhaps the gcc
> maintainers could at least consider something like
> 
> 	int __attribute((threadsafe)) fred;
> 
> ??

Not needed for recent gcc versions -- both the C11 and C++11 standards
require that different threads be permitted to independently access
different variables, where "different" means "no bits of the two variables
residing in the same byte".  From what I can see, this would mean that
a conforming pre-EV56 Alpha C11 compiler would need to use LDL_L and
STL_C to carry out 8-bit and 16-bit stores.  ;-)

							Thanx, Paul

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