Re: sched: softlockups in multi_cpu_stop

Davidlohr Bueso
2015-03-06T17:19:15Z

On Fri, 2015-03-06 at 13:32 +0100, Ingo Molnar wrote:
> * Sasha Levin <sasha.levin@oracle.com> wrote:
>
> > I've bisected this to "locking/rwsem: Check for active lock before bailing on spinning". Relevant parties Cc'ed.
>
> That would be:
>
> 1a99367023f6 ("locking/rwsem: Check for active lock before bailing on spinning")
>
> attached below.

[...]

> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 1c0d11e8ce34..e4ad019e23f5 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -298,23 +298,30 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> {
> struct task_struct *owner;
> - bool on_cpu = false;
> + bool ret = true;
>
> if (need_resched())
> return false;
>
> rcu_read_lock();
> owner = ACCESS_ONCE(sem->owner);
> - if (owner)
> - on_cpu = owner->on_cpu;
> - rcu_read_unlock();
> + if (!owner) {
> + long count = ACCESS_ONCE(sem->count);
> + /*
> + * If sem->owner is not set, yet we have just recently entered the
> + * slowpath with the lock being active, then there is a possibility
> + * reader(s) may have the lock. To be safe, bail spinning in these
> + * situations.
> + */
> + if (count & RWSEM_ACTIVE_MASK)
> + ret = false;
> + goto done;

Hmmm so the lockup would be due to this (when owner is non-nil the patch
has no effect), telling users to spin instead of sleep -- _except_ for
this condition. And when spinning we're always checking for need_resched
to be safe. So even if this function was completely bogus, we'd end up
needlessly spinning but I'm surprised about the lockup. Maybe coffee
will make things clearer.