Re: [PATCH] kthread: fix task state in kthread worker if being frozen

[Chen Yu <yu.c.chen@intel.com>]

Hi Andrew,

On 2024-08-19 at 18:58:07 -0700, Andrew Morton wrote:
> On Mon, 19 Aug 2024 22:15:51 +0800 Chen Yu <yu.c.chen@intel.com> wrote:
> 
> > It was reported that during cpu hotplug test, the following
> > error was triggered:
> > 
> >  do not call blocking ops when !TASK_RUNNING; state=1 set at kthread_worker_fn (kernel/kthread.c:?)
> >  WARNING: CPU: 1 PID: 674 at kernel/sched/core.c:8469 __might_sleep
> > 
> >  handle_bug
> >  exc_invalid_op
> >  asm_exc_invalid_op
> >  __might_sleep
> >  __might_sleep
> >  kthread_worker_fn
> >  kthread_worker_fn
> >  kthread
> >  __cfi_kthread_worker_fn
> >  ret_from_fork
> >  __cfi_kthread
> >  ret_from_fork_asm
> > 
> > Peter pointed out that there is a race condition when the kworker is being
> > frozen and falls into try_to_freeze() with TASK_INTERRUPTIBLE, which
> > triggeres the warning.
> 
> OK.  A full description of this race would be better than simply
> asserting that it exists, please.
>

OK, will write a description for this.
 
> > Fix this by explicitly set the TASK_RUNNING before entering try_to_freeze().
> 
> OK.
> 
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -848,6 +848,12 @@ int kthread_worker_fn(void *worker_ptr)
> >  	} else if (!freezing(current))
> >  		schedule();
> >  
> > +	/*
> > +	 * Explicitly set the running state in case we are being
> > +	 * frozen and skip the schedule() above. try_to_freeze()
> > +	 * expects the current task to be in the running state.
> > +	 */
> > +	__set_current_state(TASK_RUNNING);
> >  	try_to_freeze();
> >  	cond_resched();
> >  	goto repeat;
> 
> Comment is helpful, but why express in a comment that which can be
> expressed in code?
>

OK, will do in next version.

thanks,
Chenyu
 
> --- a/kernel/kthread.c~kthread-fix-task-state-in-kthread-worker-if-being-frozen
> +++ a/kernel/kthread.c
> @@ -847,6 +847,12 @@ repeat:
>  		trace_sched_kthread_work_execute_end(work, func);
>  	} else if (!freezing(current))
>  		schedule();
> +	} else {
> +		/*
> +		 * Handle the case where X happens
> +		 */
> +		__set_current_state(TASK_RUNNING);
> +	}
>  
>  	try_to_freeze();
>  	cond_resched();
> _
>