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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Dmitry Vyukov <dvyukov@google.com>
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
Subject: Re: [syzbot] INFO: rcu detected stall in syscall_exit_to_user_mode
In-Reply-To: <CACT4Y+avKp8LCS8vBdaFLXFNcNiCq3vF-8K59o7c1oy86v-ADA@mail.gmail.com>
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Date:   Wed, 15 Sep 2021 11:32:02 +0200
Message-ID: <87k0jib2wd.ffs@tglx>
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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.