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Date:   Wed, 13 Apr 2022 21:56:12 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Oleg Nesterov <oleg@redhat.com>
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: <20220413195612.GC2762@worktop.programming.kicks-ass.net>
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>
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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();
> 
> 			> +	if (wait_on_bit(&task->jobctl, JOBCTL_TRACED_XXX_BIT, TASK_KILLABLE))
> 			> +		return -EINTR;
> 			> +	// now that the tracee cleared JOBCTL_TRACED_XXX_BIT
> 			> +	// wait_task_inactive() should succeed or fail "really soon".
> 			> +	if (!wait_task_inactive(child, __TASK_TRACED))
> 			> +		return ret;
> 
> 
> 	*whoopsie* ?
> 
> >  		preempt_enable_no_resched();
> >  		freezable_schedule();
> >  		cgroup_leave_frozen(true);

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();
	}
	__set_current_state(TASK_RUNNING);

	if (!wait_task_inactive(...))


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