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Date:   Thu, 21 Apr 2022 11:46:41 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, rjw@rjwysocki.net,
        mingo@kernel.org, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de,
        bigeasy@linutronix.de, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org, tj@kernel.org,
        linux-pm@vger.kernel.org
Subject: Re: [RFC][PATCH] ptrace: Don't change __state
Message-ID: <20220421094640.GA18344@redhat.com>
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On 04/20, Eric W. Biederman wrote:
>
> I was thinking about this and I have an approach from a different
> direction.  In particular it removes the need for ptrace_freeze_attach
> and ptrace_unfreeze_attach to change __state.  Instead a jobctl
> bit is used to suppress waking up a process with TASK_WAKEKILL.

I think this can work, but we still need something like 1/5 + 2/5?

> I think this would be a good technique to completely decouple
> PREEMPT_RT from the work that ptrace_freeze_attach does.

If CONFIG_RT=y we can't rely on the ->__state check in task_is_traced(),
and wait_task_inactive() can wrongly fail if the tracee sleeps waiting
for tasklist_lock.

A couple of comments after a quick glance,

>  static void ptrace_unfreeze_traced(struct task_struct *task)
>  {
> -	if (READ_ONCE(task->__state) != __TASK_TRACED)
> +	if (!task_is_traced(task))
>  		return;
>
>  	WARN_ON(!task->ptrace || task->parent != current);
> @@ -216,13 +217,11 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
>  	 * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
>  	 * Recheck state under the lock to close this race.
>  	 */
> -	spin_lock_irq(&task->sighand->siglock);
> -	if (READ_ONCE(task->__state) == __TASK_TRACED) {
> -		if (__fatal_signal_pending(task))
> -			wake_up_state(task, __TASK_TRACED);
> -		else
> -			WRITE_ONCE(task->__state, TASK_TRACED);
> -	}
> +	spin_unlock_irq(&task->sighand->siglock);
> +	WARN_ON(!(task->jobctl & JOBCTL_DELAY_WAKEKILL));
> +	task->jobctl &= ~JOBCTL_DELAY_WAKEKILL;

We can't rely on the lockless task_is_traced() check above... probably this
is fine, but I need to re-chesk. But at least you need to remove the comment
about PTRACE_LISTEN above.

Another problem is that WARN_ON(!(task->jobctl & JOBCTL_DELAY_WAKEKILL))
doesn't look right if ignore_state in ptrace_check_attach() was true, the
tracee could stop before ptrace_unfreeze_traced().

> @@ -892,7 +891,6 @@ static int ptrace_resume(struct task_struct *child, long request,
>  	 * status and clears the code too; this can't race with the tracee, it
>  	 * takes siglock after resume.
>  	 */
> -	need_siglock = data && !thread_group_empty(current);
>  	if (need_siglock)
>  		spin_lock_irq(&child->sighand->siglock);

Hmm?

Oleg.