linux-kernel - Re: INFO: rcu detected stall in do

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Message-ID: <20181016140322.GB3121@hirez.programming.kicks-ass.net>
Date:   Tue, 16 Oct 2018 16:03:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     syzbot <syzbot+385468161961cee80c31@...kaller.appspotmail.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>, mingo@...hat.com,
        nstange@...e.de, syzkaller-bugs@...glegroups.com,
        Juri Lelli <juri.lelli@...il.com>
Subject: Re: INFO: rcu detected stall in do_idle

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?