[tip: timers/urgent] clocksource: Prevent double add_timer_on() for watchdog_timer

From: tip-bot2 for Konstantin Khlebnikov
Date: Sat Feb 01 2020 - 06:21:32 EST


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

Commit-ID: febac332a819f0e764aa4da62757ba21d18c182b
Gitweb: https://git.kernel.org/tip/febac332a819f0e764aa4da62757ba21d18c182b
Author: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
AuthorDate: Fri, 31 Jan 2020 19:08:59 +03:00
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Sat, 01 Feb 2020 11:07:56 +01:00

clocksource: Prevent double add_timer_on() for watchdog_timer

Kernel crashes inside QEMU/KVM are observed:

kernel BUG at kernel/time/timer.c:1154!
BUG_ON(timer_pending(timer) || !timer->function) in add_timer_on().

At the same time another cpu got:

general protection fault: 0000 [#1] SMP PTI of poinson pointer 0xdead000000000200 in:

__hlist_del at include/linux/list.h:681
(inlined by) detach_timer at kernel/time/timer.c:818
(inlined by) expire_timers at kernel/time/timer.c:1355
(inlined by) __run_timers at kernel/time/timer.c:1686
(inlined by) run_timer_softirq at kernel/time/timer.c:1699

Unfortunately kernel logs are badly scrambled, stacktraces are lost.

Printing the timer->function before the BUG_ON() pointed to
clocksource_watchdog().

The execution of clocksource_watchdog() can race with a sequence of
clocksource_stop_watchdog() .. clocksource_start_watchdog():

expire_timers()
detach_timer(timer, true);
timer->entry.pprev = NULL;
raw_spin_unlock_irq(&base->lock);
call_timer_fn
clocksource_watchdog()

clocksource_watchdog_kthread() or
clocksource_unbind()

spin_lock_irqsave(&watchdog_lock, flags);
clocksource_stop_watchdog();
del_timer(&watchdog_timer);
watchdog_running = 0;
spin_unlock_irqrestore(&watchdog_lock, flags);

spin_lock_irqsave(&watchdog_lock, flags);
clocksource_start_watchdog();
add_timer_on(&watchdog_timer, ...);
watchdog_running = 1;
spin_unlock_irqrestore(&watchdog_lock, flags);

spin_lock(&watchdog_lock);
add_timer_on(&watchdog_timer, ...);
BUG_ON(timer_pending(timer) || !timer->function);
timer_pending() -> true
BUG()

I.e. inside clocksource_watchdog() watchdog_timer could be already armed.

Check timer_pending() before calling add_timer_on(). This is sufficient as
all operations are synchronized by watchdog_lock.

Fixes: 75c5158f70c0 ("timekeeping: Update clocksource with stop_machine")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/158048693917.4378.13823603769948933793.stgit@buzz
---
kernel/time/clocksource.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index fff5f64..428beb6 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -293,8 +293,15 @@ static void clocksource_watchdog(struct timer_list *unused)
next_cpu = cpumask_next(raw_smp_processor_id(), cpu_online_mask);
if (next_cpu >= nr_cpu_ids)
next_cpu = cpumask_first(cpu_online_mask);
- watchdog_timer.expires += WATCHDOG_INTERVAL;
- add_timer_on(&watchdog_timer, next_cpu);
+
+ /*
+ * Arm timer if not already pending: could race with concurrent
+ * pair clocksource_stop_watchdog() clocksource_start_watchdog().
+ */
+ if (!timer_pending(&watchdog_timer)) {
+ watchdog_timer.expires += WATCHDOG_INTERVAL;
+ add_timer_on(&watchdog_timer, next_cpu);
+ }
out:
spin_unlock(&watchdog_lock);
}