linux-kernel - Re: RFC: SysRq nice-all-RT-tasks is broken

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Message-ID: <20170308115114.4e20fc73@gandalf.local.home>
Date:   Wed, 8 Mar 2017 11:51:14 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     Andrew Morton <akpm@...l.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>
Subject: Re: RFC: SysRq nice-all-RT-tasks is broken

On Wed, 8 Mar 2017 16:23:35 +0100
Laurent Dufour <ldufour@...ux.vnet.ibm.com> wrote:

> I got it on Power and on X86_64, but I guess it should happen in all
> architectures.
> Here are the steps to recreate it :
> 1. Create a RT task : sudo chrt -f 50 /bin/sleep 999999
> 2. On the console trigger the 'nice-all-RT-tasks' SYS-RQ.
> 
> The panic is triggered by the BUG_ON(in_interrupt()) introduced by this
> commit:
> 
> 66e5393a78b3 ("[PATCH] BUG() if setscheduler is called from interrupt
> context")
> 
> Since SysRq is run from the interrupt context, the panic is expected.
> 
> Looking at the code, I'm wondering if the BUG_ON() is still required in
> __sched_setscheduler(). But I'm not so confident, so requesting your
> advise here.
> 

Hmm, that commit was added in 2.6.18, and you're right, a lot has
changed since then. Have you tried removing it and running it under
lockdep, and see if it triggers any warnings?

-- Steve