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

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Date:	Mon, 14 Feb 2011 10:51:13 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>, 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

On Sat, Feb 12, 2011 at 07:47:45PM +0100, Peter Zijlstra wrote:
> On Fri, 2011-02-11 at 22:38 +0100, Peter Zijlstra wrote:
> > 
> > So why can't we make that jump_label_entry::refcount and
> > jump_label_key::state an atomic_t and be done with it? 
> 
> So I had a bit of a poke at this because I didn't quite understand why
> all that stuff was as it was. I applied both Jason's patches and then
> basically rewrote kernel/jump_label.c just for kicks ;-)
> 
> I haven't tried compiling this, let alone running it, but provided I
> didn't actually forget anything the storage per key is now 16 bytes when
> modules are disabled and 24 * (1 + mods) bytes for when they are
> enabled. The old code had 64 + 40 * mods bytes.
> 
> I still need to clean up the static_branch_else bits and look at !x86
> aside from the already mentioned bits.. but what do people think?
> 
> ---

Generally, I really like this! Its the direction I think the jump label
code should be going. The complete removal of the hash table, makes the
design a lot better and simpler. We just need to get some of the details
cleaned up, and of course we need this to compile :) But I don't see any
fundamental problems with this approach. 

Things that still need to be sorted out:

1) Since jump_label.h, are included in kernel.h, (indirectly via the
dynamic_debug.h) the atomic_t definitions could be problematic, since
atomic.h includes kernel.h indirectly...so we might need some header
magic.

2) I had some code to disallow writing to module __init section, by
setting the 'key' value to 0, after the module->init was run, but
before, the memory was freed. And then I check for a non-zero key value
when the jump label is updated. In this way we can't corrupt some random
piece of memory. I had this done via the 'MODULE_STATE_LIVE' notifier.

3) For 'jump_label_enable()' 'jump_label_disable()' in the tracepoint
code, I'm not sure that there is an enable for each disable. So i'm not
sure if a refcount would work there. But we can fix this by first
checking 'jump_label_enabled()' before calling 'jump_label_eanble()' or
jump_label_ref(). This is safe b/c the the tracepoint code is protected
using the tracepoint_mutex.

thanks,

-Jason 

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