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Date:   Sun, 28 Oct 2018 09:33:55 +0100
From:   Juri Lelli <juri.lelli@redhat.com>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        luca abeni <luca.abeni@santannapisa.it>,
        Thomas Gleixner <tglx@linutronix.de>,
        Juri Lelli <juri.lelli@gmail.com>,
        syzbot <syzbot+385468161961cee80c31@syzkaller.appspotmail.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, nstange@suse.de,
        syzkaller-bugs@googlegroups.com, henrik@austad.us,
        Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>,
        Claudio Scordino <claudio@evidence.eu.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: INFO: rcu detected stall in do_idle
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On 27/10/18 12:16, Dmitry Vyukov wrote:
> On Wed, Oct 24, 2018 at 1:03 PM, Juri Lelli <juri.lelli@redhat.com> wrote:
> >
> > On 19/10/18 22:50, luca abeni wrote:
> > > On Fri, 19 Oct 2018 13:39:42 +0200
> > > Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > > On Thu, Oct 18, 2018 at 01:08:11PM +0200, luca abeni wrote:
> > > > > Ok, I see the issue now: the problem is that the "while
> > > > > (dl_se->runtime <= 0)" loop is executed at replenishment time, but
> > > > > the deadline should be postponed at enforcement time.
> > > > >
> > > > > I mean: in update_curr_dl() we do:
> > > > >   dl_se->runtime -= scaled_delta_exec;
> > > > >   if (dl_runtime_exceeded(dl_se) || dl_se->dl_yielded) {
> > > > >           ...
> > > > >           enqueue replenishment timer at dl_next_period(dl_se)
> > > > > But dl_next_period() is based on a "wrong" deadline!
> > > > >
> > > > >
> > > > > I think that inserting a
> > > > >         while (dl_se->runtime <= -pi_se->dl_runtime) {
> > > > >                 dl_se->deadline += pi_se->dl_period;
> > > > >                 dl_se->runtime += pi_se->dl_runtime;
> > > > >         }
> > > > > immediately after "dl_se->runtime -= scaled_delta_exec;" would fix
> > > > > the problem, no?
> > > >
> > > > That certainly makes sense to me.
> > >
> > > Good; I'll try to work on this idea in the weekend.
> >
> > So, we (me and Luca) managed to spend some more time on this and found a
> > few more things worth sharing. I'll try to summarize what we have got so
> > far (including what already discussed) because impression is that each
> > point might deserve a fix or at least consideration (just amazing how a
> > simple random fuzzer thing can highlight all that :).
> 
> 1. Fuzzing finds bugs in any code. Always.
> If a code wasn't fuzzed, there are bugs.
> 
> 2. This fuzzer is not so simple ;)

Indeed! I meant that it's amazing how the fuzzer was able to forge a
relatively simple reproducer that highlighted the problem.