Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 16 Oct 2018 16:03:22 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     syzbot <syzbot+385468161961cee80c31@syzkaller.appspotmail.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        LKML <linux-kernel@vger.kernel.org>, mingo@redhat.com,
        nstange@suse.de, syzkaller-bugs@googlegroups.com,
        Juri Lelli <juri.lelli@gmail.com>
Subject: Re: INFO: rcu detected stall in do_idle
Message-ID: <20181016140322.GB3121@hirez.programming.kicks-ass.net>
References: <000000000000a4ee200578172fde@google.com>
 <alpine.DEB.2.21.1810161516430.7787@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.DEB.2.21.1810161516430.7787@nanos.tec.linutronix.de>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Oct 16, 2018 at 03:24:06PM +0200, Thomas Gleixner wrote:
> It does reproduce here but with a kworker stall. Looking at the reproducer:
> 
>   *(uint32_t*)0x20000000 = 0;
>   *(uint32_t*)0x20000004 = 6;
>   *(uint64_t*)0x20000008 = 0;
>   *(uint32_t*)0x20000010 = 0;
>   *(uint32_t*)0x20000014 = 0;
>   *(uint64_t*)0x20000018 = 0x9917;
>   *(uint64_t*)0x20000020 = 0xffff;
>   *(uint64_t*)0x20000028 = 0;
>   syscall(__NR_sched_setattr, 0, 0x20000000, 0);
> 
> which means:
> 
>   struct sched_attr {
>   	 .size		= 0,
> 	 .policy	= 6,
> 	 .flags		= 0,
> 	 .nice		= 0,
> 	 .priority	= 0,
> 	 .deadline	= 0x9917,
> 	 .runtime	= 0xffff,
> 	 .period	= 0,
>   }
> 
> policy 6 is SCHED_DEADLINE
> 
> That makes the thread hog the CPU and prevents all kind of stuff to run.
> 
> Peter, is that expected behaviour?

Sorta, just like FIFO-99 while(1);. Except we should be rejecting the
above configuration, because of the rule:

  runtime <= deadline <= period

Juri, where were we supposed to check that?