linux-kernel - Re: [patch] Re: using long instead of atomic

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Date:	Mon, 3 Mar 2008 09:31:57 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Alan Stern <stern@...land.harvard.edu>,
	Pavel Machek <pavel@....cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...l.org>,
	Zdenek Kabelac <zdenek.kabelac@...il.com>, davem@...emloft.net,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Pierre Ossman <drzeus-mmc@...eus.cx>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: [patch] Re: using long instead of atomic_t when only set/read is required

On Tue, Mar 04, 2008 at 04:16:33AM +1100, Nick Piggin wrote:
> On Tuesday 04 March 2008 02:53, Alan Cox wrote:
> > > Atomicity of reads of write for pointers and integral types (other than
> > > long long) should be documented.
> >
> > NAK.
> >
> > Atomicity of reads or writes for pointers and integral types is NOT
> > guaranteed. Gcc doesn't believe in your guarantee.
> 
> Are you sure gcc doesn't? Or is it just "C"?
> 
> Linux wouldn't work today if gcc did something non-atomic there
> (presuming you're talking about naturally aligned pointers/ints).
> It is widely used and accepted.
> 
> RCU users are far from the only places to rely on this, although
> I guess they are the main ones when it comes to assigning pointers
> atomically.

It is true that gcc can refetch pointers/ints if it runs out of registers,
which is why rcu_dereference() recently had an ACCESS_ONCE() added to it.

But such refetching cannot result in a mish-mash of two different
pointer values, confusing though it might be to the affected code.

						Thanx, Paul
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