linux-kernel - Re: [PATCH] x86: Code atomic(64)_read and atomic(64)_set in C not CPP [was Re: FRV: Implement atomic64

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Message-ID: <20090702072157.GA11596@elte.hu>
Date:	Thu, 2 Jul 2009 09:21:57 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, akpm@...ux-foundation.org,
	arnd@...db.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Code atomic(64)_read and atomic(64)_set in C not
	CPP [was Re: FRV: Implement atomic64_t]


* Paul Mackerras <paulus@...ba.org> wrote:

> Occasionally we get bugs where atomic_read or atomic_set are used 
> on atomic64_t variables or vice versa.  These bugs don't generate 
> warnings on x86 because atomic_read and atomic_set are coded as 
> macros rather than C functions, so we don't get any type-checking 
> on their arguments; similarly for atomic64_read and atomic64_set 
> in 64-bit kernels.
> 
> This converts them to C functions so that the arguments are 
> type-checked and bugs like this will get caught more easily. It 
> also converts atomic_cmpxchg and atomic_xchg, and atomic64_cmpxchg 
> and atomic64_xchg on 64-bit, so we get type-checking on their 
> arguments too.
> 
> Compiling a typical 64-bit x86 config, this generates no new 
> warnings, and the vmlinux text is 86 bytes smaller.
> 
> Signed-off-by: Paul Mackerras <paulus@...ba.org>

Thanks Paul!

> ---
> Linus Torvalds writes:
> 
> > Btw, Ingo: I looked at the x86-32 versions to be sure, and noticed a 
> > couple of buglets:
> > 
> >  - atomic64_xchg uses "atomic_read()". Sure, it happens to work, since 
> >    the "atomic_read()" is not type-safe, and gets a non-atomic 64-bit 
> >    read, but that looks really really bogus.
> > 
> >    It _should_ use __atomic64_read(), and the 64-bit versions should use a 
> >    different counter name ("counter64"?) or we should use an inline 
> >    function for atomic_read(), so that the type safety issue gets fixed.
> 
> I did this patch a few weeks ago (before the merge window) and 
> sent it to Ingo, Thomas & Peter, but it seems to have got lost.

Yeah, as i noted back then off-list i didnt take it due to it 
causing a criss-cross merge:

| > Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
|
| Nice - could someone please remind us later in the merge window to 
| have a look at this again? Right now this needs to go into 
| perfcounters/core - but i'd like to avoid having to do too many 
| cross-changes there.

Linus reminded us ;-)

        Ingo
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