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Date:   Fri, 5 Jul 2019 15:24:26 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     "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>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: INFO: rcu detected stall in ext4_write_checks

On Thu, Jun 27, 2019 at 12:47 AM Theodore Ts'o <tytso@....edu> wrote:
>
> More details about what is going on.  First, it requires root, because
> one of that is required is using sched_setattr (which is enough to
> shoot yourself in the foot):
>
> sched_setattr(0, {size=0, sched_policy=0x6 /* SCHED_??? */, sched_flags=0, sched_nice=0, sched_priority=0, sched_runtime=2251799813724439, sched_deadline=4611686018427453437, sched_period=0}, 0) = 0
>
> This is setting the scheduler policy to be SCHED_DEADLINE, with a
> runtime parameter of 2251799.813724439 seconds (or 26 days) and a
> deadline of 4611686018.427453437 seconds (or 146 *years*).  This means
> a particular kernel thread can run for up to 26 **days** before it is
> scheduled away, and if a kernel reads gets woken up or sent a signal,
> no worries, it will wake up roughly seven times the interval that Rip
> Van Winkle spent snoozing in a cave in the Catskill Mountains (in
> Washington Irving's short story).
>
> We then kick off a half-dozen threads all running:
>
>    sendfile(fd, fd, &pos, 0x8080fffffffe);
>
> (and since count is a ridiculously large number, this gets cut down to):
>
>    sendfile(fd, fd, &pos, 2147479552);
>
> Is it any wonder that we are seeing RCU stalls?   :-)

+Peter, Ingo for sched_setattr and +Paul for rcu

First of all: is it a semi-intended result of a root (CAP_SYS_NICE)
doing local DoS abusing sched_setattr? It would perfectly reasonable
to starve other processes, but I am not sure about rcu. In the end the
high prio process can use rcu itself, and then it will simply blow
system memory by stalling rcu. So it seems that rcu stalls should not
happen as a result of weird sched_setattr values. If that is the case,
what needs to be fixed? sched_setattr? rcu? sendfile?

If this is semi-intended, the only option I see is to disable
something in syzkaller: sched_setattr entirely, or drop CAP_SYS_NICE,
or ...? Any preference either way?

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