linux-kernel - Re: [PATCH] livepatch: fix race between enabled_store() and klp_unregister

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Message-ID: <20151201155337.GD12513@treble.redhat.com>
Date:	Tue, 1 Dec 2015 09:53:37 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Li Bin <huawei.libin@...wei.com>, sjenning@...hat.com,
	jikos@...nel.org, vojtech@...e.com, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, guohanjun@...wei.com,
	dingtianhong@...wei.com, xiexiuqi@...wei.com,
	zhouchengming1@...wei.com, Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH] livepatch: fix race between enabled_store() and
 klp_unregister_patch()

On Tue, Dec 01, 2015 at 09:50:23AM +0100, Jiri Slaby wrote:
> On 12/01/2015, 02:11 AM, Josh Poimboeuf wrote:
> > When I try to recreate something similar by putting a delay in
> > enabled_store(), klp_free_patch() just sleeps on its call to
> > kobject_put() until enabled_store() returns.  The unregister stack looks
> > like:
> > 
> >   [<ffffffff812e966b>] __kernfs_remove+0x1fb/0x380
> >   [<ffffffff812ea273>] kernfs_remove+0x23/0x40
> >   [<ffffffff812ec601>] sysfs_remove_dir+0x51/0x80
> >   [<ffffffff81407fb8>] kobject_del+0x18/0x50
> >   [<ffffffff8140804a>] kobject_release+0x5a/0x190
> >   [<ffffffff81407f27>] kobject_put+0x27/0x50
> 
> What about _put outside of klp_mutex in klp_unregister_patch (and maybe
> the other _put's as well)? Plus Li Bin's patch.

This approach sounds the best to me.  I think all _put's for the patch
kobj need to be outside the mutex.  There's also a _put for the patch
kobj in the klp_init_patch() error path which needs to be moved out.

I think the rest of the _put's (for object and func kobjs) are fine as
they are, because they don't have corresponding sysfs functions which
get the mutex.

-- 
Josh
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