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

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Message-Id: <20081107102148.6eb53dea.akpm@linux-foundation.org>
Date:	Fri, 7 Nov 2008 10:21:48 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	dhowells@...hat.com, nico@....org, torvalds@...ux-foundation.org,
	mingo@...e.hu, a.p.zijlstra@...llo.nl,
	linux-kernel@...r.kernel.org, ralf@...ux-mips.org,
	benh@...nel.crashing.org, paulus@...ba.org, davem@...emloft.net,
	mingo@...hat.com, tglx@...utronix.de, rostedt@...dmis.org,
	linux-arch@...r.kernel.org
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

On Fri, 7 Nov 2008 13:00:41 -0500
Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> * 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.

Referring to include/linux/tracepoint.h:DEFINE_TRACE()?

It does look a bit fragile.  Does every .c file which included
include/trace/block.h get a copy of __tracepoint_block_rq_issue,
whether or not it used that tracepoint? Hopefully not.

> 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).

I'm unsure of the requirements here.  Do you _want_ each call to
trace_block_rq_issue() to share some in-memory state?  If so then yes,
there's a problem with calls to trace_block_rq_issue() from within
separate compilation units.

otoh, if all calls to trace_block_rq_issue() are supposed to have
independent state (which seems to be the case) then that could be
addressed by making trace_block_rq_issue() a macro which defines
static storage, as cnt32_to_63() shouldn't have done ;)


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