Messages in this thread | | | Subject | Re: softlockups in multi_cpu_stop | From | Davidlohr Bueso <> | Date | Fri, 06 Mar 2015 18:26:24 -0800 |
| |
On Sat, 2015-03-07 at 10:10 +0800, Ming Lei wrote: > On Sat, Mar 7, 2015 at 10:07 AM, Davidlohr Bueso <dave@stgolabs.net> wrote: > > On Sat, 2015-03-07 at 09:55 +0800, Ming Lei wrote: > >> On Fri, 06 Mar 2015 14:15:37 -0800 > >> Davidlohr Bueso <dave@stgolabs.net> wrote: > >> > >> > On Fri, 2015-03-06 at 13:12 -0800, Jason Low wrote: > >> > > In owner_running() there are 2 conditions that would make it return > >> > > false: if the owner changed or if the owner is not running. However, > >> > > that patch continues spinning if there is a "new owner" but it does not > >> > > take into account that we may want to stop spinning if the owner is not > >> > > running (due to getting rescheduled). > >> > > >> > So you're rationale is that we're missing this need_resched: > >> > > >> > while (owner_running(sem, owner)) { > >> > /* abort spinning when need_resched */ > >> > if (need_resched()) { > >> > rcu_read_unlock(); > >> > return false; > >> > } > >> > } > >> > > >> > Because the owner_running() would return false, right? Yeah that makes > >> > sense, as missing a resched is a bug, as opposed to our heuristics being > >> > so painfully off. > >> > > >> > Sasha, Ming (Cc'ed), does this address the issues you guys are seeing? > >> > >> For the xfstest lockup, what matters is that the owner isn't running, since > >> the following simple change does fix the issue: > > > > I much prefer Jason's approach, which should also take care of the > > issue, as it includes the !owner->on_cpu stop condition to stop > > spinning. > > But the check on owner->on_cpu should be moved outside the loop > because new owner can be scheduled out too, right?
That's not what this is about. New lock _owners_ need to worry about burning cycles trying to acquire the lock ;)
> >> > >> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > >> index 06e2214..5e08705 100644 > >> --- a/kernel/locking/rwsem-xadd.c > >> +++ b/kernel/locking/rwsem-xadd.c > >> @@ -358,8 +358,9 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner) > >> } > >> rcu_read_unlock(); > >> > >> - if (READ_ONCE(sem->owner)) > >> - return true; /* new owner, continue spinning */ > >> + owner = READ_ONCE(sem->owner); > >> + if (owner && owner->on_cpu) > >> + return true;
So if I'm understanding this right, your patch works because you add another on_cpu check and at this point we could very well have sem->owner == owner -- such that owner_running return false for the same reason in the first place! So Jason's patch takes on the issue directly by never allowing ups to reach this point.
Thanks, Davidlohr
| |