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

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Message-ID: <20160504033619.5osteklgal3ixcbo@treble>
Date:	Tue, 3 May 2016 22:36:19 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jiri Kosina <jikos@...nel.org>
Cc:	Miroslav Benes <mbenes@...e.cz>, 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 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.

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.

-- 
Josh