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Subject: Re: [patch] spinlocks: remove 'volatile'
From: Arjan van de Ven <arjan@infradead.org>
To: "J.A." =?ISO-8859-1?Q?Magall=F3n?= <jamagallon@ono.com>
Cc: "linux-os (Dick Johnson)" <linux-os@analogic.com>,
       Ingo Molnar <mingo@elte.hu>, Linus Torvalds <torvalds@osdl.org>,
       Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
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> > I did not talk about memory barriers. In fact, barrier() is NOT a memory
> > barrier. It's a compiler optimization barrier!
> > 
> 
> // Read 10 samples from 2 A/D converters.
> 
> int*	ina;
> int	a[10];
> int*	inb;
> int	b[10];
> 
> for (int i=0; i<10; i++)
> {
> 	a[i] = *ina;
> 	barrier();
> 	b[i] = *inb;
> }
> 
> The barrier prevents the compiler of translating this to:
> 
> for (int i=0; i<10; i++)
> {
> 	b[i] = *inb;
> 	a[i] = *ina;
> }
> 
> or even to:
> 
> for (int i=0; i<10; i++)
> 	a[i] = *ina;
> for (int i=0; i<10; i++)
> 	b[i] = *inb;
> 
> but does not prevent it to do this:

yes it does. It's a full optimization barrier; the compiler assumes all
register and memory content has changed from before the barrier(), and
it will start "fresh".