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Subject: Re: [patch] spinlocks: remove 'volatile'
From: Arjan van de Ven <arjan@infradead.org>
To: Avi Kivity <avi@argo.co.il>
Cc: Linus Torvalds <torvalds@osdl.org>, Mark Lord <lkml@rtr.ca>,
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Date: Sat, 08 Jul 2006 11:51:48 +0200
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On Sat, 2006-07-08 at 12:20 +0300, Avi Kivity wrote:
> Arjan van de Ven wrote:
> >
> > >
> > > It could be argued that gcc's implementation of volatile is wrong, and
> > > that gcc should add the appropriate serializing instructions before and
> > > after volatile accesses.
> > >
> > > Of course, that would make volatile even more suboptimal, but at least
> > > correct.
> >
> > with PCI, and the PCI posting rules, there is no "one" serializing
> > instruction, you need to know the specifics of the device in question to
> > cause the flush. So at least there is no universal possible
> > implementation of volatile as you suggest ;-)
> >
> 
> A serializing volatile makes it possible to write portable code to 
> access pci mmio.  You'd just follow a write with a read or whatever the 
> rules say.

yeah except that the compiler cannot know what to read; reading back the
same memory location is NOT correct nor safe. It's device specific, for
some devices it'll be safe, for others you have to read some OTHER
location.