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SubjectRe: timers: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected
On Tue, Jan 12, 2016 at 03:03:27PM -0500, Sasha Levin wrote:
> [ 3408.703754] Call Trace:

> [ 3408.733192] rcu_read_unlock_special (kernel/rcu/tree_plugin.h:503)
> [ 3408.735155] __rcu_read_unlock (kernel/rcu/update.c:223)
> [ 3408.736090] __lock_timer (include/linux/rcupdate.h:495 include/linux/rcupdate.h:930 kernel/time/posix-timers.c:709)

I'm thinking this is one of those magic preemptible RCU bits..

---
kernel/time/posix-timers.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 31d11ac9fa47..09e28733e725 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -701,17 +701,25 @@ static struct k_itimer *__lock_timer(timer_t timer_id, unsigned long *flags)
if ((unsigned long long)timer_id > INT_MAX)
return NULL;

+ /*
+ * One of the few rules of preemptible RCU is that one cannot do
+ * rcu_read_unlock() while holding a scheduler (or nested) lock when
+ * part of the read side critical section was irqs-enabled -- see
+ * rcu_read_unlock_special().
+ */
+ local_irq_safe(*flags);
rcu_read_lock();
timr = posix_timer_by_id(timer_id);
if (timr) {
- spin_lock_irqsave(&timr->it_lock, *flags);
+ spin_lock(&timr->it_lock);
if (timr->it_signal == current->signal) {
rcu_read_unlock();
return timr;
}
- spin_unlock_irqrestore(&timr->it_lock, *flags);
+ spin_unlock(&timr->it_lock);
}
rcu_read_unlock();
+ local_irq_restore(*flags);

return NULL;
}
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