Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

Mathieu Desnoyers
2008-11-07T18:34:01Z

* Andrew Morton (akpm@linux-foundation.org) wrote:
> On Fri, 7 Nov 2008 13:00:41 -0500
> Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
>
> > * Andrew Morton (akpm@linux-foundation.org) wrote:
> > > On Fri, 07 Nov 2008 17:10:00 +0000 David Howells <dhowells@redhat.com> wrote:
> > >
> > > >
> > > > Andrew Morton <akpm@linux-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.
>

No, __tracepoint_block_rq_issue is only instanciated if the static
inline function is used. One instance per use.

> > 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 ;)
>

They could share the same data, given it *has* to be the same. I'll
try to fix this.

Mathieu

>

--
Mathieu Desnoyers
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