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

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Message-ID: <BLU0-SMTP618D94A58771D9533EB21896C80@phx.gbl>
Date:	Thu, 10 Mar 2011 09:11:39 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	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,
	ddaney@...iumnetworks.com, 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 Wed, 2011-03-09 at 15:47 -0500, Jason Baron wrote:
> > Hi,
> > 
> > Re-fresh of updates against latest -tip tree.
> 
> Thanks Jason,
> 
> I started looking at them, I should have comments tomorrow (if I have
> any comments ;)
> 
> > 
> > I've tried to split this update up somewhat, but I've only succeeded to split
> > out the dynamic debug bits. The interface changes and re-write are quite
> > intertwined.
> > 
> > I believe this update should address all the comments from the previous posting
> > except for Mathieu's request for a section of jump label pointers that point to
> > the jump label structures (since the compiler might leave gaps in the jump label
> > structures).
> 
> The jump label structures is a list of 3 pointers, correct? I doubt that
> gcc would place any holes in it as they are all aligned by natural word
> size.
> 

Hi Steven,

Can you explain what would prevent gcc from aligning these 3 pointers
(total of 24 bytes on 64-bit architectures) on 32-bytes ? Also, could
you point out what would refrain the linker from aligning the start of
object sections on the next 32-bytes (thus power of two) address
multiple ?

I think we need to be a bit more strict in our interpretation of what
guarantee gcc/ld provide and don't provide with respect to section and
structure alignment.

As it stands now, the section alignment of jump labels looks half-broken
on most architectures, and this *is* a big deal. I would really like to
see a patch for this (it can be a separate patch) going in for .39.

Thank you,

Mathieu

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