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SubjectRe: timers: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected
On Tue, Jan 12, 2016 at 09:18:56PM +0100, Peter Zijlstra wrote:
> 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..

Hmmm... Looking back at Sasha's original email, RCU doesn't have much
choice about making ->wait_lock HARDIRQ-irq-unsafe, since it acquires
it via a call to rt_mutex_lock(), which cannot be invoked with irqs
disabled. In fact, it seems a bit odd to acquire something named
->wait_lock with irqs disabled.

That said...

> ---
> 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();

If ->it_lock is ever acquired while one of the rq or pi locks was held,
Peter's patch is needed.

It is just that I am not seeing what I would expect to see in Sasha's
lockdep splat if that were the case.

Thanx, Paul

> 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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