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From: Hillf Danton <hdanton@sina.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: syzbot <syzbot+d1ae9c954f73570e7b58@syzkaller.appspotmail.com>,
	linux-kernel@vger.kernel.org,
	netdev@vger.kernel.org,
	syzkaller-bugs@googlegroups.com,
	Johannes Berg <johannes.berg@intel.com>,
	Kalle Valo <kvalo@codeaurora.org>,
	linux-wireless@vger.kernel.org
Subject: Re: [syzbot] [kernel?] WARNING in hrtimer_forward (2)
Date: Wed, 10 Apr 2024 06:51:39 +0800
Message-Id: <20240409225139.1556-1-hdanton@sina.com>
In-Reply-To: <87a5m27r08.ffs@tglx>
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On Tue, 09 Apr 2024 12:46:31 +0200 Thomas Gleixner <tglx@linutronix.de>
> On Mon, Apr 08 2024 at 19:46, syzbot wrote:
> > ------------[ cut here ]------------
> > WARNING: CPU: 1 PID: 11239 at kernel/time/hrtimer.c:1053 hrtimer_forward+0x210/0x2d0 kernel/time/hrtimer.c:1053
> > RIP: 0010:hrtimer_forward+0x210/0x2d0 kernel/time/hrtimer.c:1053
> > Call Trace:
> >  <IRQ>
> >  hrtimer_forward_now include/linux/hrtimer.h:355 [inline]
> >  mac80211_hwsim_beacon+0x192/0x1f0 drivers/net/wireless/virtual/mac80211_hwsim.c:2335
> >  __run_hrtimer kernel/time/hrtimer.c:1692 [inline]
> >  __hrtimer_run_queues+0x595/0xd00 kernel/time/hrtimer.c:1756
> >  hrtimer_run_softirq+0x19a/0x2c0 kernel/time/hrtimer.c:1773
> >  __do_softirq+0x2bc/0x943 kernel/softirq.c:554
> >  invoke_softirq kernel/softirq.c:428 [inline]
> >  __irq_exit_rcu+0xf2/0x1c0 kernel/softirq.c:633
> >  irq_exit_rcu+0x9/0x30 kernel/softirq.c:645
> >  instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1043 [inline]
> >  sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1043
> >  </IRQ>
> 
> The code which arms the timer is not serialized against the callback so
> the following can happen:
> 
>     CPU1                        CPU2
> 
>     __run_hrtimer(timer)
>                                 if (!hrtimer_queued(timer))
>                                         hrtimer_start(timer);
> 
>        hrtimer_forward(timer)
>          WARN_ON(hrtimer_queued(timer)
> 
> This really wants to use hrtimer_active() which takes the running
> callback into account.
> 
Then the race window is still open with hrtimer_active() and serialization
outside hrtimer is needed.

	CPU1				CPU2
	---				---
					if (!hrtimer_is_active(timer))
	if (!hrtimer_is_active(timer))
		hrtimer_start(timer);
	__run_hrtimer(timer)
						hrtimer_start(timer);
	hrtimer_forward(timer)
	  WARN_ON(hrtimer_queued(timer))