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

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Message-ID: <alpine.LFD.0.999.0708181853210.29105@enigma.security.iitk.ac.in>
Date:	Sat, 18 Aug 2007 19:06:33 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Christoph Lameter <clameter@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Paul Mackerras <paulus@...ba.org>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	heiko.carstens@...ibm.com, horms@...ge.net.au,
	linux-kernel@...r.kernel.org, rpjday@...dspring.com, ak@...e.de,
	netdev@...r.kernel.org, cfriesen@...tel.com,
	akpm@...ux-foundation.org, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org, zlynx@....org, schwidefsky@...ibm.com,
	Chris Snook <csnook@...hat.com>, davem@...emloft.net,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures



On Fri, 17 Aug 2007, Linus Torvalds wrote:

> On Sat, 18 Aug 2007, Satyam Sharma wrote:
> > 
> > No code does (or would do, or should do):
> > 
> > 	x.counter++;
> > 
> > on an "atomic_t x;" anyway.
> 
> That's just an example of a general problem.
> 
> No, you don't use "x.counter++". But you *do* use
> 
> 	if (atomic_read(&x) <= 1)
> 
> and loading into a register is stupid and pointless, when you could just 
> do it as a regular memory-operand to the cmp instruction.

True, but that makes this a bad/poor code generation issue with the
compiler, not something that affects the _correctness_ of atomic ops if
"volatile" is used for that counter object (as was suggested), because
we'd always use the atomic_inc() etc primitives to do increments, which
are always (should be!) implemented to be atomic.


> And as far as the compiler is concerned, the problem is the 100% same: 
> combining operations with the volatile memop.
> 
> The fact is, a compiler that thinks that
> 
> 	movl mem,reg
> 	cmpl $val,reg
> 
> is any better than
> 
> 	cmpl $val,mem
> 
> is just not a very good compiler.

Absolutely, this is definitely a bug report worth opening with gcc. And
what you've said to explain this previously sounds definitely correct --
seeing "volatile" for any access does appear to just scare the hell out
of gcc and makes it generate such (poor) code.


Satyam
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