linux-kernel - Re: [RFC patch 08/18] cnt32_to_63 should use smp

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Date:	Fri, 7 Nov 2008 13:00:41 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Howells <dhowells@...hat.com>, Nicolas Pitre <nico@....org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, Ralf Baechle <ralf@...ux-mips.org>,
	benh@...nel.crashing.org, paulus@...ba.org,
	David Miller <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arch@...r.kernel.org
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

* Andrew Morton (akpm@...ux-foundation.org) wrote:
> On Fri, 07 Nov 2008 17:10:00 +0000 David Howells <dhowells@...hat.com> wrote:
> 
> > 
> > Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > > I'd expect it to behave in the same way as it would if the function was
> > > implemented out-of-line.
> > > 
> > > But it occurs to me that the modrobe-doesnt-work thing would happen if
> > > the function _is_ inlined anyway, so we won't be doing that.
> > > 
> > > Whatever.  Killing this many puppies because gcc may do something so
> > > bizarrely wrong isn't justifiable.
> > 
> > With gcc, you get one instance of the static variable from inside a static
> > (inline or outofline) function per .o file that invokes it, and these do not
> > merge even though they're common symbols.  I asked around and the opinion
> > seems to be that this is correct C.  I suppose it's the equivalent of cutting
> > and pasting a function between several files - why should the compiler assume
> > it's the same function in each?
> > 
> 
> OK, thanks, I guess that makes sense.  For static inline.  I wonder if
> `extern inline' or plain old `inline' should change it.
> 
> It's one of those things I hope I never need to know about, but perhaps
> we do somewhere have static storage in an inline.  Wouldn't surprise
> me, and I bet that if we do, it's a bug.

Tracepoints actually use that. It could be changed so they use :

DECLARE_TRACE() (in include/trace/group.h)
DEFINE_TRACE()  (in the appropriate kernel c file)
trace_somename(); (in the code)

instead. That would actually make more sense and remove the need for
multiple declarations when the same tracepoint name is used in many
spots (this is a problem kmemtrace has, it generates a lot of tracepoint
declarations).

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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