timer: Read jiffies once when forwarding base clk (e430d802) · Commits · 戴 / test

The timer delayed for more than 3 seconds warning was triggered during testing. Workqueue: events_unbound sched_tick_remote RIP: 0010:sched_tick_remote+0xee/0x100 ... Call Trace: process_one_work+0x18c/0x3a0 worker_thread+0x30/0x380 kthread+0x113/0x130 ret_from_fork+0x22/0x40 The reason is that the code in collect_expired_timers() uses jiffies unprotected: if (next_event > jiffies) base->clk = jiffies; As the compiler is allowed to reload the value base->clk can advance between the check and the store and in the worst case advance farther than next event. That causes the timer expiry to be delayed until the wheel pointer wraps around. Convert the code to use READ_ONCE() Fixes: ("timers: Optimize collect_expired_timers() for NOHZ") Signed-off-by:

Li RongQing <lirongqing@baidu.com> Signed-off-by:

Liang ZhiCheng <liangzhicheng@baidu.com> Signed-off-by:

Thomas Gleixner <tglx@linutronix.de> Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/1568894687-14499-1-git-send-email-lirongqing@baidu.com

kernel/time/timer.c

+5 −3

Original line number	Original line	Diff line number	Diff line
	@@ -1678,24 +1678,26 @@ void timer_clear_idle(void)
	static int collect_expired_timers(struct timer_base *base,		static int collect_expired_timers(struct timer_base *base,
	struct hlist_head *heads)		struct hlist_head *heads)
	{		{
			unsigned long now = READ_ONCE(jiffies);

	/*		/*
	* NOHZ optimization. After a long idle sleep we need to forward the		* NOHZ optimization. After a long idle sleep we need to forward the
	* base to current jiffies. Avoid a loop by searching the bitfield for		* base to current jiffies. Avoid a loop by searching the bitfield for
	* the next expiring timer.		* the next expiring timer.
	*/		*/
	if ((long)(jiffies - base->clk) > 2) {		if ((long)(now - base->clk) > 2) {
	unsigned long next = __next_timer_interrupt(base);		unsigned long next = __next_timer_interrupt(base);

	/*		/*
	* If the next timer is ahead of time forward to current		* If the next timer is ahead of time forward to current
	* jiffies, otherwise forward to the next expiry time:		* jiffies, otherwise forward to the next expiry time:
	*/		*/
	if (time_after(next, jiffies)) {		if (time_after(next, now)) {
	/*		/*
	* The call site will increment base->clk and then		* The call site will increment base->clk and then
	* terminate the expiry loop immediately.		* terminate the expiry loop immediately.
	*/		*/
	base->clk = jiffies;		base->clk = now;
	return 0;		return 0;
	}		}
	base->clk = next;		base->clk = next;

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