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Date: Mon, 15 Jul 2019 15:46:51 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Dmitry Vyukov <dvyukov@...gle.com> Cc: "Paul E. McKenney" <paulmck@...ux.ibm.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 Mon, Jul 15, 2019 at 03:33:11PM +0200, Dmitry Vyukov wrote: > On Mon, Jul 15, 2019 at 3:29 PM Peter Zijlstra <peterz@...radead.org> wrote: > > > > 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. > > Since we are racing with emails, could you suggest a 100% safe > parameters? Because I only hear people saying "safe", "sane", > "well-behaving" :) > If we move the check to user-space, it does not mean that we can get > away without actually defining what that means. Right, well, that's part of the problem. I think Paul just did the reverse math and figured that 95% of X must not be larger than my watchdog timeout and landed on 14 seconds. I'm thinking 4 seconds (or rather 4.294967296) would be a very nice number. > Now thinking of this, if we come up with some simple criteria, could > we have something like a sysctl that would allow only really "safe" > parameters? I suppose we could do that, something like: sysctl_deadline_period_{min,max}. I'll have to dig back a bit on where we last talked about that and what the problems where. For one, setting the min is a lot harder, but I suppose we can start at TICK_NSEC or something.
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