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

[Peter Zijlstra]

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.