linux-kernel - Re: [PATCH v14 05/11] livepatch: Simplify API by removing registration step

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Date:   Fri, 14 Dec 2018 08:27:10 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jiri Kosina <jikos@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
        Jason Baron <jbaron@...mai.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v14 05/11] livepatch: Simplify API by removing
 registration step

On Fri, Dec 14, 2018 at 11:02:32AM +0100, Petr Mladek wrote:
> On Thu 2018-12-13 16:46:25, Josh Poimboeuf wrote:
> > Now that we can't re-enable a patch, I wonder if we really need both the
> > 'patch->enabled' and 'klp_target_state' variables?
> > 
> > A patch is now always enabled, unless it's in transition, in which case
> > its 'enabled' state is the same as 'klp_target_state'.
> >
> > For example I wonder if we could get rid of 'klp_target_state', since it
> > should be the same as 'klp_transition_patch->enabled'.
> 
> There are some catches:
> 
> 1. klp_update_patch_state() can be called anywhere and anytime. We
>    would add yet another race-sensitive code if we access the flag
>    via a pointer.

True.

> 2. patch->enabled is bool while klp_target_state is triple state.
>    The argument is that KLP_UNDEFINED helps to catch bugs.
> 
> 
> > Or alternatively we could get rid of 'patch->enabled', since it should
> > be the same as
> > 
> > 	patch == klp_transition_patch ? klp_target_state : true
> 
> This might solve the first catch but not the 2nd one.

It could WARN_ON if it's KLP_UNDEFINED when it's not supposed to be.

> Not to say that it is much harder to read.

We could put it in a klp_patch_enabled(patch) wrapper.

> > Of course this could be a follow-on cleanup patch, which could be done
> > in the future, so as not to hold up the merging of these patches
> > anymore.
> 
> Yes, please. This is controversial, non-trivial, and can wait.

No problem.

-- 
Josh