Re: sched: softlockups in multi_cpu_stop

Sasha Levin
2015-03-06T18:03:41Z

On 03/06/2015 12:19 PM, Davidlohr Bueso wrote:
>> 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.

There's always the possibility that bisect went wrong. I did it twice, but
since I don't have a sure way of reproducing it I was basing my good/bad
decisions on whether I saw it within a reasonable amount of time.

I can go redo that again if you suspect that that commit is not the cause.

Thanks,
Sasha