linux-kernel - Re: [PATCH 0/2] jump label: update for .39

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Date:	Thu, 10 Mar 2011 16:22:14 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	David Daney <ddaney@...iumnetworks.com>,
	Jason Baron <jbaron@...hat.com>, peterz@...radead.org,
	hpa@...or.com, 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,
	michael@...erman.id.au, linux-kernel@...r.kernel.org,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH 0/2] jump label: update for .39

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Thu, 2011-03-10 at 14:25 -0500, Steven Rostedt wrote:
> > On Thu, 2011-03-10 at 10:57 -0800, David Daney wrote:
> > > On 03/10/2011 10:53 AM, Steven Rostedt wrote:
> > > > On Thu, 2011-03-10 at 10:47 -0800, David Daney wrote:
> > > >
> > > >> The alignment requested by the assembler will have to satisfy *all* the
> > > >> requested alignments, so manually forcing everything to .align 8 (or
> > > >> .align 4 for 32-bit) should ensure that the linker doesn't put in any holes.
> > > >
> > > > I would agree with the assessment although, I don't know that it is
> > > > documented anywhere that this is what happens. As the previous "bug"
> > > > with the trace_events was solved by me adding .align(4) everywhere, I
> > > > would think that .align(sizeof(long)) would work here too.
> > > >
> > > > It may be a good ideal to force this alignment, and not add wasted
> > > > space. If anything, if this (hypothetical) bug appears, it will most
> > > > likely show up as a crash on boot up. I'm not too concerned about it.
> > > >
> > > 
> > > If the linker put in gratuitous holes, things like  __ex_table would 
> > > break too.
> > 
> > Again, as I have said (although I said extable not __ex_table), there
> > seems to be no problem when the data is of a power of 2 as well. As
> > __ex_table size is a power of 2. We just don't know if the linker will
> > add holes when the size is something other than power of 2.
> 
> Anyway, I think the best thing for now is to have Jason add
> the .align(sizeof(long)) in the inline assembly for all locations and be
> done with it.

You seem to be contradicting yourself here. I'm concerned about having
"structures" of a size not power of two. Can we simply either

- Add a padding element at the end
or
- use .align 4*sizeof(long) at the beginning

to make sure the linker won't put any holes when it puts objects 
together ?

Thanks,

Mathieu

> 
> -- Steve
> 
> 

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