Messages in this thread | | | Date | Thu, 9 Apr 2015 09:47:36 -0700 | Subject | Re: [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner() | From | Linus Torvalds <> |
| |
On Thu, Apr 9, 2015 at 12:53 AM, Ingo Molnar <mingo@kernel.org> wrote: > > The point is to generally unify the 'out' paths - i.e. to merge it > with the rcu_read_unlock() as well, so that we have really simple > gotos and only a single exit path.
Maybe just have the rcu read-locking be done in the *caller* (possibly through using just a helper wrapper function that does nothing but the locking), so that you can just do a simple "return false" in the function itself.
That said, it worries me a bit that we do that spinning while holding the RCU read lock in the first place. Yes, we stop spinning if "need_resched()" is set, but what effect - if any - does all of this have on RCU latency? If somebody is waiting for a RCU grace period, I'm not seeing that setting need-resched...
At least with CONFIG_PREEMPT_RCU, the read-unlock is *not* just doing a preempt-disable, so it's not necessarily just about need_resched(). It does all the magic with 'rcu_read_unlock_special.s' too..
Adding Paul. From a RCU locking standpoint, the thing is basically (not the real code, edited down):
rcu_read_lock(); while (sem->owner == owner) { if (!owner->on_cpu || need_resched()) break; cpu_relax_lowlatency(); } rcu_read_unlock();
so we busy-loop while holding the RCU read lock while
sem->owner == owner && owner->on_cpu && !need_resched()
is true. That is usually not very long, but we've already had watchdogs go off when we get this wrong, so..
Paul, comments? Are there particular latency concerns wrt CONFIG_PREEMPT_RCU here? Or am I just being silly?
Linus
| |