Re: [PATCH v4 10/12] ptrace: Don't change __state

[Oleg Nesterov <oleg@xxxxxxxxxx>]

On 05/05, Eric W. Biederman wrote:
>
>  static void ptrace_unfreeze_traced(struct task_struct *task)
>  {
> -	if (READ_ONCE(task->__state) != __TASK_TRACED)
> -		return;
> -
> -	WARN_ON(!task->ptrace || task->parent != current);
> +	unsigned long flags;
>  
>  	/*
> -	 * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
> -	 * Recheck state under the lock to close this race.
> +	 * The child may be awake and may have cleared
> +	 * JOBCTL_PTRACE_FROZEN (see ptrace_resume).  The child will
> +	 * not set JOBCTL_PTRACE_FROZEN or enter __TASK_TRACED anew.
>  	 */
> -	spin_lock_irq(&task->sighand->siglock);
> -	if (READ_ONCE(task->__state) == __TASK_TRACED) {
> +	if (lock_task_sighand(task, &flags)) {

But I still think that a lockless

	if (!(task->jobctl & JOBCTL_PTRACE_FROZEN))
		return;

check at the start of ptrace_unfreeze_traced() makes sense to avoid
lock_task_sighand() if possible.

And ptrace_resume() can probably clear JOBCTL_PTRACE_FROZEN along with
JOBCTL_TRACED to make this optimization work better. The same for
ptrace_signal_wake_up().

Oleg.