linux-kernel - Re: livepatch: Avoid possible race when releasing the patch

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Message-ID: <20160523213540.GC12954@packer-debian-8-amd64.digitalocean.com>
Date:	Mon, 23 May 2016 17:35:41 -0400
From:	Jessica Yu <jeyu@...hat.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	jpoimboe@...hat.com, mbenes@...e.cz, jikos@...nel.org,
	jslaby@...e.cz, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, huawei.libin@...wei.com,
	minfei.huang@...oo.com
Subject: Re: livepatch: Avoid possible race when releasing the patch

+++ Petr Mladek [23/05/16 17:54 +0200]:
>There was a long discussion about a possible race with sysfs, kobjects
>when removing an unused livepatch, see
>https://lkml.kernel.org/g/%3C1462190242-24731-1-git-send-email-mbenes@suse.cz%3E
>
>This patch set tries to implement what looked the most preferred solution
>from the discussion. I did my best to keep the patch definition simple.
>But I am not super happy with the result.
>
>I send the current state before I spent even more time on different
>approaches.
>
>I personally think that we might get better result if we declare
>some limited structures, define them statically and then copy all
>data into the final structures in a single call. I did not implement
>this because it was weird on the first look but I am not sure now.
>
>But even more I would prefer the solution with the completion.
>It is already used by the module framework. It does not look
>that hacky to me after all.

Hi Petr, thanks a lot for the RFC and for exploring this possible
solution. I haven't reviewed the patches thoroughly yet, but at first
glance I admit that I did not think through how much this approach
would complicate the livepatch API, and the new intermediary functions
do seem like overkill in response to the original kobject problem..

I looked at how the module loader used the completion, and in fact
it is used to remedy a nearly identical problem with
DEBUG_KOBJ_RELEASE (see commit 942e443 "Fix mod->mkobj.kobj
potentially freed too early"), and Miroslav's original solution pretty
much took the same approach. We could even mirror that approach and
have something like klp_kobject_put() (much like mod_kobject_put()) to
package up the kobject_put/wait_for_completion calls, but that is
purely a matter of taste.

Anyway, I am just beginning to lean towards the completion solution
again (sorry for jumping back and forth :-/), but we can play with
this patchset a bit more and see if we can come up with something
reasonable.

Thanks,
Jessica