linux-kernel - Re: [PATCH v4.1 2/2] livepatch: force transition to finish

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Date:   Mon, 18 Dec 2017 14:23:40 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Jason Baron <jbaron@...mai.com>
cc:     jpoimboe@...hat.com, jeyu@...nel.org, jikos@...nel.org,
        pmladek@...e.com, lpechacek@...e.cz, pavel@....cz,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4.1 2/2] livepatch: force transition to finish

On Fri, 15 Dec 2017, Jason Baron wrote:

> On 11/22/2017 05:29 AM, Miroslav Benes wrote:
> > If a task sleeps in a set of patched functions uninterruptedly, it could
> > block the whole transition indefinitely.  Thus it may be useful to clear
> > its TIF_PATCH_PENDING to allow the process to finish.
> > 
> > Admin can do that now by writing to force sysfs attribute in livepatch
> > sysfs directory. TIF_PATCH_PENDING is then cleared for all tasks and the
> > transition can finish successfully.
> > 
> > Important note! Administrator should not use this feature without a
> > clearance from a patch distributor. It must be checked that by doing so
> > the consistency model guarantees are not violated. Removal (rmmod) of
> > patch modules is permanently disabled when the feature is used. It
> > cannot be guaranteed there is no task sleeping in such module.
> > 
> > Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> > Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > Reviewed-by: Petr Mladek <pmladek@...e.com>
> > ---
> >  Documentation/ABI/testing/sysfs-kernel-livepatch | 14 ++++++++++
> >  Documentation/livepatch/livepatch.txt            | 18 ++++++++++--
> >  kernel/livepatch/core.c                          | 30 ++++++++++++++++++++
> >  kernel/livepatch/transition.c                    | 35 +++++++++++++++++++++++-
> >  kernel/livepatch/transition.h                    |  1 +
> >  5 files changed, 95 insertions(+), 3 deletions(-)
> 
> ....
> 
> > +
> > +/*
> > + * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
> > + * existing transition to finish.
> > + *
> > + * NOTE: klp_update_patch_state(task) requires the task to be inactive or
> > + * 'current'. This is not the case here and the consistency model could be
> > + * broken. Administrator, who is the only one to execute the
> > + * klp_force_transitions(), has to be aware of this.
> > + */
> > +void klp_force_transition(void)
> > +{
> > +	struct task_struct *g, *task;
> > +	unsigned int cpu;
> > +
> > +	pr_warn("forcing remaining tasks to the patched state\n");
> > +
> > +	read_lock(&tasklist_lock);
> > +	for_each_process_thread(g, task)
> > +		klp_update_patch_state(task);
> > +	read_unlock(&tasklist_lock);
> > +
> > +	for_each_possible_cpu(cpu)
> > +		klp_update_patch_state(idle_task(cpu));
> > +
> > +	klp_forced = true;
> > +}
> 
> I had a question on this bit. If say cpu 0 executes
> klp_force_transition(void), right up until klp_forced is set to true,
> and then cpu 1 does klp_complete_transition() (since all threads have
> the correct state), wouldn't it be possible then for
> klp_complete_transition() to not see klp_forced set to true, and thus
> the module could be potentially removed even though it was forced?

Yes, you're right. That could happen.
 
> If so, I think that the force path just needs to be set before the
> threads are updated (as below). I don't think that the
> klp_complete_transition() needs the corresponding rmb, b/c there is
> sufficient ordering there already (although it would deserve a comment).

Or we can take klp_mutex in force_store() (kernel/livepatch/core.c) and be 
done with it once and for all. The problem is exactly what Petr predicted 
and I refused to have klp_mutex here just because it may have fixed 
theoretical issue. 

Petr, Josh, what do you think?

Miroslav

> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index be5bfa5..cca6a3a 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -671,6 +671,15 @@ void klp_force_transition(void)
> 
>         pr_warn("forcing remaining tasks to the patched state\n");
> 
> +       klp_forced = true;
> +
> +       /*
> +        * ensure that if klp_complete_transition() sees that all
> +        * the threads have been updated to desired task->patch_state
> +        * that we also see klp_forced = true;
> +        */
> +       smp_wmb();
> +
>         read_lock(&tasklist_lock);
>         for_each_process_thread(g, task)
>                 klp_update_patch_state(task);
> @@ -678,6 +687,4 @@ void klp_force_transition(void)
> 
>         for_each_possible_cpu(cpu)
>                 klp_update_patch_state(idle_task(cpu));
> -
> -       klp_forced = true;
>  }
>