linux-kernel - Re: [RFC PATCH] livepatch: allow removal of a disabled patch

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Message-ID: <20160504131423.5yqvie2zy67jspak@treble>
Date:	Wed, 4 May 2016 08:14:23 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Jiri Kosina <jikos@...nel.org>, jeyu@...hat.com, pmladek@...e.com,
	jslaby@...e.cz, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, huawei.libin@...wei.com,
	minfei.huang@...oo.com
Subject: Re: [RFC PATCH] livepatch: allow removal of a disabled patch

On Wed, May 04, 2016 at 01:58:47PM +0200, Miroslav Benes wrote:
> On Tue, 3 May 2016, Josh Poimboeuf wrote:
> 
> > On Tue, May 03, 2016 at 09:39:48PM -0500, Josh Poimboeuf wrote:
> > > On Wed, May 04, 2016 at 12:31:12AM +0200, Jiri Kosina wrote:
> > > > On Tue, 3 May 2016, Josh Poimboeuf wrote:
> > > > 
> > > > > > 1. Do we really need a completion? If I am not missing something
> > > > > > kobject_del() always waits for sysfs callers to leave thanks to kernfs
> > > > > > active protection.
> > > > > 
> > > > > What do you mean by "kernfs active protection"?  I see that
> > > > > kernfs_remove() gets the kernfs_mutex lock, but I can't find anywhere
> > > > > that a write to a sysfs file uses that lock.
> > > > > 
> > > > > I'm probably missing something...
> > > > 
> > > > I don't want to speak on Miroslav's behalf, but I'm pretty sure that what 
> > > > he has on mind is per-kernfs_node active refcounting kernfs does (see 
> > > > kernfs_node->active, and especially it's usage in __kernfs_remove()).
> > > > 
> > > > More specifically, execution of store() and show() sysfs callbacks is 
> > > > guaranteed (by kernfs) to happen with that particular attribute's active 
> > > > reference held for reading (and that makes it impossible for that 
> > > > attribute to vanish prematurely).
> > > 
> > > Thanks, that makes sense.
> > > 
> > > So what exactly is the problem the completion is trying to solve?  Is it
> > > to ensure that the kobject has been cleaned up before it returns to the
> > > caller, in case the user wants to call klp_register() again after
> > > unregistering?
> > > 
> > > If so, that's quite an unusual use case which I think we should just
> > > consider unsupported.  In fact, if you try to do it, kobject_init()
> > > complains loudly because kobj->state_initialized is still 1 because
> > > kobjects aren't meant to be reused like that.
> > 
> > ... and now I realize the point is actually to prevent the caller from
> > freeing klp_patch before kobject_cleanup() runs.
> 
> Exactly. Sorry I was so brief.
> 
> > So yeah, it looks like we need the completion in case
> > CONFIG_DEBUG_KOBJECT_RELEASE is enabled.
> > 
> > Or alternatively we could convert patch->kobj to be dynamically
> > allocated instead of embedded in klp_patch.
> 
> But that wouldn't help, would it? Dynamic kobjects registers generic 
> release function dynamic_kobj_release() and that's it. We're in the same 
> situation. I have got a feeling that dynamic kobjects are only for trivial 
> cases.

But the patch release doesn't need to do anything, right?

> Moreover we use container_of() several times in the code and that does not
> work with dynamically allocated kobjects.
> 
> Anyway I am really confused now. When I read changelog of c817a67ecba7 
> ("kobject: delayed kobject release: help find buggy drivers") all makes 
> perfect sense. But isn't our situation somewhat special, because we have 
> refcounts completely under control? So we know that once we call 
> kobject_put() we can let a patch go... I must be missing something.
> 
> It does not make sense to introduce completion just to satisfy a feature 
> which was introduced to debug general cases.

I think our situation is "special" because klp_patch and its embedded
kobject have separate lifetimes.  We have a kobject, which we own, which
is embedded in a klp_patch struct, which we don't own.

If I understand correctly, normally when you release a kobject, the
containing struct gets freed.  But we can't do that here because the
caller allocated the klp_patch.

So I really get the feeling that a dynamic kobject would be more
appropriate here.

That said, the sysfs and kobject stuff always throws me for a loop.  So
take what I'm saying with several grains of salt.

-- 
Josh