linux-kernel - Re: WARNING in hrtimer

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Message-ID: <CACT4Y+YMTiR5bKKEV2vfSfrqSjoGjjsfWfhreBsNdxQ+yDpS2g@mail.gmail.com>
Date:   Wed, 30 Sep 2020 09:26:59 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Hillf Danton <hdanton@...a.com>,
        syzbot <syzbot+ca740b95a16399ceb9a5@...kaller.appspotmail.com>,
        David Miller <davem@...emloft.net>, hchunhui@...l.ustc.edu.cn,
        Julian Anastasov <ja@....bg>, James Morris <jmorris@...ei.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Johannes Berg <johannes.berg@...el.com>
Subject: Re: WARNING in hrtimer_forward

On Mon, Sep 28, 2020 at 8:36 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Tue, Sep 29 2020 at 01:11, Hillf Danton wrote:
> > On Mon, 28 Sep 2020 18:13:42 +0200 Thomas Gleixner wrote:
> >> So the timer was armed at some point and then the expiry which does the
> >> forward races with the ioctl which starts the timer. Lack of
> >> serialization or such ...
> >
> > To make syzbot happy, s/hrtimer_is_queued/hrtimer_active/ can close
> > that race but this warning looks benign.
>
> Why only make sysbot happy? It's clearly an issue and the warning is not
> benign simply because forwarding a queued timer is an absolute NONO.
> timers (both timer_list and hrtimer) need external synchronization.

Oh, Thomas, it's so nice to hear this interpretation of things among
all the cases where people only fixing tools and making them happy :)
Don't make my tools happy! They don't need that! :)