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Date:   Fri, 7 Apr 2023 16:49:09 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Hillf Danton <hdanton@sina.com>
Cc:     Marco Elver <elver@google.com>,
        syzbot <syzbot+3b14b2ed9b3d06dcaa07@syzkaller.appspotmail.com>,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: WARNING in timer_wait_running
Message-ID: <ZDAtZWrly6Hzieip@lothringen>
References: <000000000000566d5405ae2f6f70@google.com>
 <20230407134620.1034-1-hdanton@sina.com>
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On Fri, Apr 07, 2023 at 09:46:20PM +0800, Hillf Danton wrote:
> On 5 Apr 2023 23:07:24 +0200 Marco Elver <elver@google.com>
> > On Mon, Aug 31, 2020 at 10:07AM -0700, syzbot wrote:
> > > Hello,
> > > 
> > > syzbot found the following issue on:
> > > 
> > > HEAD commit:    1127b219 Merge tag 'fallthrough-fixes-5.9-rc3' of git://gi..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1501768e900000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=978db74cb30aa994
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=3b14b2ed9b3d06dcaa07
> > 
> > Dashboard has recent reports (also below) and reproducer (also attached).
> 
> My 2c, with PREEMPT_RT enabled it simply waits by taking timer->it_lock.
> 
> --- upstream/kernel/time/posix-cpu-timers.c
> +++ y/kernel/time/posix-cpu-timers.c
> @@ -1613,6 +1613,21 @@ static int thread_cpu_timer_create(struc
>  	return posix_cpu_timer_create(timer);
>  }
>  
> +static void posix_cpu_timer_wait_running(struct k_itimer *timer)
> +{
> +#ifdef CONFIG_PREEMPT_RT
> +	int stop = 0;
> +
> +	while (!stop) {
> +		spin_lock(&timer->it_lock);
> +		stop = timer->it.cpu.firing == 0;
> +		spin_unlock(&timer->it_lock);
> +	}

No, because there is a whole lot of preemptible area with timer->it_lock
not held between the time ctmr->firing is set to 1, and the actual handling
of that timer that holds the lock.

So no priority inheritance in that case.

There has to be a lock between the time it is set to 1 and the handling
of the timer.