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Date:   Tue, 5 Nov 2019 18:59:34 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Oleg Nesterov <oleg@redhat.com>
cc:     Florian Weimer <fweimer@redhat.com>, Shawn Landden <shawn@git.icu>,
        libc-alpha@sourceware.org, linux-api@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Deepa Dinamani <deepa.kernel@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Keith Packard <keithp@keithp.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: handle_exit_race && PF_EXITING
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On Tue, 5 Nov 2019, Thomas Gleixner wrote:

> On Tue, 5 Nov 2019, Oleg Nesterov wrote:
> > On 11/05, Thomas Gleixner wrote:
> > >
> > > Out of curiosity, what's the race issue vs. robust list which you are
> > > trying to solve?
> > 
> > Off-topic, but this reminds me...
> > 
> > 	#include <sched.h>
> > 	#include <assert.h>
> > 	#include <unistd.h>
> > 	#include <syscall.h>
> > 
> > 	#define FUTEX_LOCK_PI		6
> > 
> > 	int main(void)
> > 	{
> > 		struct sched_param sp = {};
> > 
> > 		sp.sched_priority = 2;
> > 		assert(sched_setscheduler(0, SCHED_FIFO, &sp) == 0);
> > 
> > 		int lock = vfork();
> > 		if (!lock) {
> > 			sp.sched_priority = 1;
> > 			assert(sched_setscheduler(0, SCHED_FIFO, &sp) == 0);
> > 			_exit(0);
> > 		}
> > 
> > 		syscall(__NR_futex, &lock, FUTEX_LOCK_PI, 0,0,0);
> > 		return 0;
> > 	}
> > 
> > this creates the unkillable RT process spinning in futex_lock_pi() on
> > a single CPU machine (or you can use taskset).
> 
> Uuurgh.

But staring more at it. That's a scheduler bug.

parent	    	    	child

 set FIFO prio 2

 fork()	         ->	set FIFO prio 1
 		 	 sched_setscheduler(...)
			   return from syscall		<= BUG

 		 	_exit()

When the child lowers its priority from 2 to 1, then the parent _must_
preempt the child simply because the parent is now the top priority task on
that CPU. Child should never reach exit before the parent blocks on the
futex.

Peter?

What's even more disturbing is that even with that bug happening the child
is able to set PF_EXITING, but not PF_EXITPIDONE. That doesn't make sense.

Thanks,

	tglx