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

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Date:	Mon, 14 Feb 2011 16:57:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jbaron@...hat.com>
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 Mon, 2011-02-14 at 10:51 -0500, Jason Baron wrote:
> 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.

Yes, I remember running into that when I did the jump_label_ref stuff,
some head-scratching is in order there.

> 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.

AH! I wondered what that was about.. that wouldn't work now since we
actually rely on iter->key to remain what it was.

> 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.

Right,.. I hadn't considered people using it like that, but like you
said, that should be easily fixed.
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