Re: [RFC][PATCH 7/7] livepatch,context_tracking: Avoid disturbing NOHZ_FULL tasks

Petr Mladek
2021-09-24T07:34:07Z

On Thu 2021-09-23 15:28:56, Peter Zijlstra wrote:
> On Thu, Sep 23, 2021 at 03:14:48PM +0200, Petr Mladek wrote:
>
> > IMHO, this is not safe:
> >
> > CPU0 CPU1
> >
> > klp_check_task(A)
> > if (context_tracking_state_cpu(task_cpu(task)) == CONTEXT_USER)
> > goto complete;
> >
> > clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
> >
> > # task switching to kernel space
> > klp_update_patch_state(A)
> > if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
> > //false
> >
> > # calling kernel code with old task->patch_state
> >
> > task->patch_state = klp_target_state;
> >
> > BANG: CPU0 sets task->patch_state when task A is already running
> > kernel code on CPU1.
>
> Why is that a problem? That is, who actually cares about
> task->patch_state ? I was under the impression that state was purely for
> klp itself, to track which task has observed the new state.

It is the other way. The patch_state is used in klp_ftrace_handler()
to decide which code must be used (old or new).

The state must change only when the given task is _not_ using
any patched function. Hence we do it when:

+ no patched function is on the stack (needed primary for kthreads)
+ entering/leaving kernel (reliable way for user space)

See "Consistency model" in Documentation/livepatch/livepatch.rst
for more details.

Best Regards,
Petr