linux-kernel - Re: [PATCH v2 1/2] livepatch/module: Apply patch when loaded module is unformed

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Date:	Fri, 6 Mar 2015 16:35:49 +0100
From:	Petr Mladek <pmladek@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Seth Jennings <sjenning@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Miroslav Benes <mbenes@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	mingo@...nel.org, mathieu.desnoyers@...icios.com, oleg@...hat.com,
	paulmck@...ux.vnet.ibm.com, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, andi@...stfloor.org,
	rostedt@...dmis.org, tglx@...utronix.de
Subject: Re: [PATCH v2 1/2] livepatch/module: Apply patch when loaded module
 is unformed

On Fri 2015-03-06 08:54:23, Josh Poimboeuf wrote:
> On Fri, Mar 06, 2015 at 03:00:13PM +0100, Petr Mladek wrote:
> > This brings me back to the original idea with that boolean that
> > marks the state before and after the coming notifier (module_init).
> > We could use a bitfield instead of the two booleans when requested.
> 
> Yeah, that would work.  Though I think one boolean is enough?
> 
> For example, just have a mod->klp_live which is initialized to false,
> with its value set in the COMING notifier, and cleared in the GOING
> notifier.

Great optimization. I will use it.

> > Alternative solutions:
> > 
> > + reject new patches when a module is coming; this is ugly
> > 
> > + wait with adding new patch until the module leaves the COMING state;
> > this might be dangerous or complicated; we would need to leave
> > kgr_lock in the middle of the patch registration to avoid a deadlock
> > with klp_module_init(); also we might need a waitqueue for each module
> > which seems to be even bigger overhead than the two booleans
> > 
> > + always register/enable new patches and fix up the potential mess
> > (registered patches order) in klp_module_init(); This is nasty and
> > prone to regressions in the future development;
> > 
> > + add another MODULE_STATE where the kallsyms are visible but the
> > module is not used yet; this looks to complex; the module states are
> > checked on "many" locations
> > 
> > 
> > I will wait with v3 over the weekend. I hope that it will bring fresh
> > mind. Sigh, if I could have slept more with the baby twins.
> 
> Good luck!  In my experience, an entire weekend with babies is far from
> restful ;-)

Yeah, you are right :-)

Thanks for review and hint.

Best Regards,
Petr
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