Received-SPF: pass (google.com: domain of linux-wireless-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <000000000000eaacf005ca975d1a@google.com> <20210831074532.2255-1-hdanton@sina.com>
 <20210914123726.4219-1-hdanton@sina.com> <87v933b3wf.ffs@tglx>
 <CACT4Y+Yd3pEfZhRUQS9ymW+sQZ4O58Dz714xSqoZvdKa_9s2oQ@mail.gmail.com>
 <87mtoeb4hb.ffs@tglx> <CACT4Y+avKp8LCS8vBdaFLXFNcNiCq3vF-8K59o7c1oy86v-ADA@mail.gmail.com>
 <87k0jib2wd.ffs@tglx>
In-Reply-To: <87k0jib2wd.ffs@tglx>
From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 16 Sep 2021 11:24:33 +0200
Message-ID: <CACT4Y+ZCRiwobf0rGUoMiUEi=6Eoxvvgxxv-c+AhH=7U6M3LXQ@mail.gmail.com>
Subject: Re: [syzbot] INFO: rcu detected stall in syscall_exit_to_user_mode
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Hillf Danton <hdanton@sina.com>,
        syzbot <syzbot+0e964fad69a9c462bc1e@syzkaller.appspotmail.com>,
        linux-kernel@vger.kernel.org, paulmck@kernel.org,
        syzkaller-bugs@googlegroups.com,
        Peter Zijlstra <peterz@infradead.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Johannes Berg <johannes.berg@intel.com>,
        Kalle Valo <kvalo@codeaurora.org>,
        linux-wireless@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, 15 Sept 2021 at 11:32, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Wed, Sep 15 2021 at 11:14, Dmitry Vyukov wrote:
> > On Wed, 15 Sept 2021 at 10:57, Thomas Gleixner <tglx@linutronix.de> wrote:
> >> That made me actually look at that mac80211_hwsim callback again.
> >>
> >>         hrtimer_forward(&data->beacon_timer, hrtimer_get_expires(timer),
> >>                         ns_to_ktime(bcn_int * NSEC_PER_USEC));
> >>
> >> So what this does is really wrong because it tries to schedule the timer
> >> on the theoretical periodic timeline. Which goes really south once the
> >> timer is late or the callback execution took longer than the
> >> period. Hypervisors scheduling out a VCPU at the wrong place will do
> >> that for you nicely.
> >
> > Nice!
> >
> > You mentioned that hrtimer_run_queues() may not return. Does it mean
> > that it can just loop executing the same re-armed callback again and
> > again? Maybe then the debug check condition should be that
> > hrtimer_run_queues() runs the same callback more than N times w/o
> > returning?
>
> Something like that.

I've filed https://bugzilla.kernel.org/show_bug.cgi?id=214429 so that
it's not lost. Thanks.