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Date:   Thu, 14 Apr 2022 13:54:12 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     rjw@rjwysocki.net, mingo@kernel.org, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de,
        ebiederm@xmission.com, bigeasy@linutronix.de,
        Will Deacon <will@kernel.org>, linux-kernel@vger.kernel.org,
        tj@kernel.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT
Message-ID: <20220414115410.GA32752@redhat.com>
References: <20220412114421.691372568@infradead.org>
 <20220412114853.842942162@infradead.org>
 <20220413132451.GA27281@redhat.com>
 <20220413185704.GA30360@redhat.com>
 <20220413185909.GB30360@redhat.com>
 <20220413192053.GY2731@worktop.programming.kicks-ass.net>
 <20220413195612.GC2762@worktop.programming.kicks-ass.net>
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On 04/13, Peter Zijlstra wrote:
>
> On Wed, Apr 13, 2022 at 09:20:53PM +0200, Peter Zijlstra wrote:
> > On Wed, Apr 13, 2022 at 08:59:10PM +0200, Oleg Nesterov wrote:
> >
> >
> > > +		// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
> > > +		// wrong, needs siglock
> > > +		current->jobctl &= ~JOBCTL_TRACED_XXX;
> > > +		wake_up_bit(&current->jobctl, ~JOBCTL_TRACED_XXX_BIT);
> > 		  __wake_up_common_lock()
> > 		    spin_lock_irqsave()
> > 		      current_save_and_set_rtlock_wait_state();

OOPS, thanks.

> Something that might work; since there's only the one ptracer, is to
> instead do something like:
>
> 	current->jobctl &= ~JOBCTL_TRACED_XXX; // siglock
> 	if (current->ptrace)
> 		wake_up_process(current->parent);
> 	preempt_enable_no_resched();
> 	schedule();
>
>
> vs
>
> 	for (;;) {
> 		set_current_state(TASK_UNINTERRUPTIBLE);
> 		if (!(p->jobctl & JOBCTL_TRACED_XXX))
> 			break;
> 		schedule();

Yes, thanks... this is racy, see below, but probably fixeable.

> ptrace_detach() needs some additional magic as well I think, but this
> might just work.

I don't think so, JOBCTL_TRACED_XXX must be always cleared in ptrace_stop()
and ptrace_detach() implies ptrace_check_attach().


Lets forget about the proble above for the moment. There is another problem
with my patch,

	if (!(child->ptrace && child->parent == current))
		return ret;

this check is racy without tasklist, we can race with another task attaching
to our natural child (so that child->parent == current), ptrace_attach() sets
task->ptrace = flags first and changes child->parent after that.

In this case:

	if (ignore_state)
		return 0;

this is just wrong,

	if (wait_on_bit(&task->jobctl, JOBCTL_TRACED_XXX_BIT, TASK_KILLABLE))
		return -EINTR;

this is fine,

	if (!wait_task_inactive(child, __TASK_TRACED))

this not right too. wait_task_inactive() can loop "forever" doing schedule_hrtimeout()
if the actual debugger stops/resumes the tracee continuously. This is pure theoretical,
but still.

And this also means that the code above needs some changes too, we can rely on
wake_up_process(current->parent).

OK, let me think about it. Thanks!

Oleg.