Subject: Re: [PATCH 0/2] jump label: update for .39
From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Jason Baron <jbaron@redhat.com>, peterz@infradead.org, hpa@zytor.com,
        mingo@elte.hu, tglx@linutronix.de, andi@firstfloor.org,
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        fweisbec@gmail.com, avi@redhat.com, davem@davemloft.net,
        sam@ravnborg.org, ddaney@caviumnetworks.com, michael@ellerman.id.au,
        linux-kernel@vger.kernel.org, Ralf Baechle <ralf@linux-mips.org>
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Date: Thu, 10 Mar 2011 10:38:24 -0500
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On Thu, 2011-03-10 at 09:11 -0500, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.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 ?

Maybe it would be just easier to add another long ;)

Seriously, it would. Then it would be 32 bytes on 64bit and 16 bytes on
32bit. Then I guess we can have our guarantee without doing a large
change to have this indirect pointer and still waste sizeof(long) bytes
in having it.

Just insert a long "Reserved" word.

-- Steve

> 
> 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
> 


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