Re: [syzbot] [kernel?] WARNING in hrtimer_forward (2)

Hillf Danton
2024-04-09T22:52:46Z

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))