linux-ext4 - Re: INFO: rcu detected stall in ext4_write

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Message-ID: <20190715132911.GG3419@hirez.programming.kicks-ass.net>
Date:   Mon, 15 Jul 2019 15:29:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>, Theodore Ts'o <tytso@....edu>,
        syzbot <syzbot+4bfbbf28a2e50ab07368@...kaller.appspotmail.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        David Miller <davem@...emloft.net>, eladr@...lanox.com,
        Ido Schimmel <idosch@...lanox.com>,
        Jiri Pirko <jiri@...lanox.com>,
        John Stultz <john.stultz@...aro.org>,
        linux-ext4@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: INFO: rcu detected stall in ext4_write_checks

On Sun, Jul 14, 2019 at 11:49:15AM -0700, Paul E. McKenney wrote:
> On Sun, Jul 14, 2019 at 05:48:00PM +0300, Dmitry Vyukov wrote:
> > But short term I don't see any other solution than stop testing
> > sched_setattr because it does not check arguments enough to prevent
> > system misbehavior. Which is a pity because syzkaller has found some
> > bad misconfigurations that were oversight on checking side.
> > Any other suggestions?
> 
> Keep the times down to a few seconds?  Of course, that might also
> fail to find interesting bugs.

Right, if syzcaller can put a limit on the period/deadline parameters
(and make sure to not write "-1" to
/proc/sys/kernel/sched_rt_runtime_us) then per the in-kernel
access-control should not allow these things to happen.