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

Peter Zijlstra
2021-09-23T13:32:59Z

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.