linux-kernel - Re: [PATCH v3] livepatch: Don't block removal of patches that are safe to unload

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Date:   Wed, 16 Mar 2022 16:01:02 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Chengming Zhou <zhouchengming@...edance.com>, jpoimboe@...hat.com,
        jikos@...nel.org, joe.lawrence@...hat.com,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        songmuchun@...edance.com, qirui.001@...edance.com
Subject: Re: [PATCH v3] livepatch: Don't block removal of patches that are
 safe to unload

On Wed 2022-03-16 15:48:25, Miroslav Benes wrote:
> On Sat, 12 Mar 2022, Chengming Zhou wrote:
> 
> > module_put() is not called for a patch with "forced" flag. It should
> > block the removal of the livepatch module when the code might still
> > be in use after forced transition.
> > 
> > klp_force_transition() currently sets "forced" flag for all patches on
> > the list.
> > 
> > In fact, any patch can be safely unloaded when it passed through
> > the consistency model in KLP_UNPATCHED transition.
> > 
> > By other words, the "forced" flag must be set only for livepatches
> 
> s/By/In/
> 
> > that are being removed. In particular, set the "forced" flag:
> > 
> >   + only for klp_transition_patch when the transition to KLP_UNPATCHED
> >     state was forced.
> > 
> >   + all replaced patches when the transition to KLP_PATCHED state was
> >     forced and the patch was replacing the existing patches.
> > 
> > index 5683ac0d2566..7f25a5ae89f6 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -641,6 +641,18 @@ void klp_force_transition(void)
> >  	for_each_possible_cpu(cpu)
> >  		klp_update_patch_state(idle_task(cpu));
> >  
> > -	klp_for_each_patch(patch)
> > -		patch->forced = true;
> > +	/*
> > +	 * Only need to set forced flag for the transition patch
> > +	 * when force transition to KLP_UNPATCHED state, but
> > +	 * have to set forced flag for all replaced patches
> > +	 * when force atomic replace transition.
> > +	 */
> 
> How about something like
> 
> /*
>  * Set forced flag for patches being removed, which is the transition
>  * patch in KLP_UNPATCHED state or all replaced patches when forcing
>  * the atomic replace transition.
>  */

Or just the first sentence:

	/* Set forced flag for patches being removed */

The rest is visible from the code.

Either version works for me. If we agree on it then I update
the text when pushing the patch.

Best Regards,
Petr