linux-kernel - Re: [PATCH 0/2] jump label: 2.6.38 updates

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Date:	Fri, 11 Feb 2011 17:32:49 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Jason Baron <jbaron@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>, hpa@...or.com,
	rostedt@...dmis.org, mingo@...e.hu, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	avi@...hat.com, davem@...emloft.net, sam@...nborg.org,
	ddaney@...iumnetworks.com, michael@...erman.id.au,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates

* Jason Baron (jbaron@...hat.com) wrote:
> On Fri, Feb 11, 2011 at 05:20:25PM -0500, Mathieu Desnoyers wrote:
> > * Peter Zijlstra (peterz@...radead.org) wrote:
> > > On Fri, 2011-02-11 at 16:13 -0500, Mathieu Desnoyers wrote:
> > > > 
> > > > Thoughts ? 
> > > 
> > >  #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
> > > +
> > > +struct jump_label_key {
> > > +       void *ptr;
> > > +};
> > > 
> > >  struct jump_label_entry {
> > >         struct hlist_node hlist;
> > >         struct jump_entry *table;
> > > -       int nr_entries;
> > >         /* hang modules off here */
> > >         struct hlist_head modules;
> > >         unsigned long key;
> > > +       u32 nr_entries;
> > > +       int refcount;
> > >  };
> > > 
> > > #else
> > > 
> > > +struct jump_label_key {
> > > +       int state;
> > > +};
> > > 
> > > #endif
> > > 
> > > So why can't we make that jump_label_entry::refcount and
> > > jump_label_key::state an atomic_t and be done with it?
> > > 
> > > Then the enabled case uses if (atomic_inc_return(&key->ptr->refcount) ==
> > > 1), and the disabled atomic_inc(&key->state).
> > > 
> > 
> > OK, by "enabled" you mean #if defined(CC_HAVE_ASM_GOTO) &&
> > defined(CONFIG_JUMP_LABEL), and "disabled", the #else.
> > 
> > I guess the only downside is the extra volatile for the atomic_read for
> > the fallback case, which is not really much of problem realistically
> > speaking: anyway, the volatile is a good thing to have in the fallback
> > case to force the compiler to re-read the variable. Let's go with your
> > idea.
> > 
> > Thanks,
> > 
> > Mathieu
> > 
> 
> ok, I'll try and re-spin the interface based around atomic_t, if we are all
> agreed...there was also a circular dependency issue with atomic.h including
> kernel.h which included jump_label.h, and that was why we had a separate,
> jump_label_ref.h header file, but hopefully I can be resolve that in a clean
> way.

See spinlocks ?

jump_label_types.h (structure definitions, includes types.h,
                    included from kernel.h)
jump_label.h (prototypes, inline functions, includes atomic.h)

Thanks,

Mathieu

> 
> thanks,
> 
> -Jason

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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