The following commit has been merged into the timers/urgent branch of tip:

Commit-ID: e430d802d6a3aaf61bd3ed03d9404888a29b9bf9 Gitweb: https://git.kernel.org/tip/e430d802d6a3aaf61bd3ed03d9404888a29b9bf9 Author: Li RongQing lirongqing@baidu.com AuthorDate: Thu, 19 Sep 2019 20:04:47 +08:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Thu, 19 Sep 2019 17:50:11 +02:00

timer: Read jiffies once when forwarding base clk

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: 236968383cf5 ("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...

--- kernel/time/timer.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 0e315a2..4820823 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -1678,24 +1678,26 @@ void timer_clear_idle(void) static int collect_expired_timers(struct timer_base *base, struct hlist_head *heads) { + unsigned long now = READ_ONCE(jiffies); + /* * NOHZ optimization. After a long idle sleep we need to forward the * base to current jiffies. Avoid a loop by searching the bitfield for * the next expiring timer. */ - if ((long)(jiffies - base->clk) > 2) { + if ((long)(now - base->clk) > 2) { unsigned long next = __next_timer_interrupt(base);

/* * If the next timer is ahead of time forward to current * 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 * terminate the expiry loop immediately. */ - base->clk = jiffies; + base->clk = now; return 0; } base->clk = next;