linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Date:	Wed, 15 Aug 2007 13:17:48 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Satyam Sharma <satyam@...radead.org>
Cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	horms@...ge.net.au, Stefan Richter <stefanr@...6.in-berlin.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	rpjday@...dspring.com, netdev@...r.kernel.org, ak@...e.de,
	cfriesen@...tel.com, Heiko Carstens <heiko.carstens@...ibm.com>,
	jesper.juhl@...il.com, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, zlynx@....org,
	clameter@....com, schwidefsky@...ibm.com,
	Chris Snook <csnook@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

On Thu, Aug 16, 2007 at 01:24:42AM +0530, Satyam Sharma wrote:
> [ The Cc: list scares me. Should probably trim it. ]

Trim away!  ;-)

> On Wed, 15 Aug 2007, Paul E. McKenney wrote:
> 
> > On Wed, Aug 15, 2007 at 08:31:25PM +0200, Segher Boessenkool wrote:
> > > >>How does the compiler know that msleep() has got barrier()s?
> > > >
> > > >Because msleep_interruptible() is in a separate compilation unit,
> > > >the compiler has to assume that it might modify any arbitrary global.
> > > 
> > > No; compilation units have nothing to do with it, GCC can optimise
> > > across compilation unit boundaries just fine, if you tell it to
> > > compile more than one compilation unit at once.
> > 
> > Last I checked, the Linux kernel build system did compile each .c file
> > as a separate compilation unit.
> > 
> > > What you probably mean is that the compiler has to assume any code
> > > it cannot currently see can do anything (insofar as allowed by the
> > > relevant standards etc.)
> 
> I think this was just terminology confusion here again. Isn't "any code
> that it cannot currently see" the same as "another compilation unit",
> and wouldn't the "compilation unit" itself expand if we ask gcc to
> compile more than one unit at once? Or is there some more specific
> "definition" for "compilation unit" (in gcc lingo, possibly?)

This is indeed my understanding -- "compilation unit" is whatever the
compiler looks at in one go.  I have heard the word "module" used for
the minimal compilation unit covering a single .c file and everything
that it #includes, but there might be a better name for this.

							Thanx, Paul
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