Messages in this thread | | | From | Joel Fernandes <> | Date | Wed, 18 Mar 2020 20:47:18 -0400 | Subject | Re: RCU use of swait |
| |
On Thu, Mar 5, 2020 at 3:11 AM Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote: > > On 2020-03-04 16:35:26 [-0800], Paul E. McKenney wrote: > > Hello! > Hi Paul, > > > Or is there some other reason why {S,}RCU needs to use swait that I > > am forgetting? > > swait can be used in hardirq-context (on RT) that is in a section within > local_irq_save() / raw_spin_lock() and so. > wait on the hand uses spinlock_t which can not be used in this context. > So if you have no users which fall into that category then you could > move back to wait.h.
In RCU, there are some truly-atomic code sections containing an swake_upXX() call, which would be considered atomic also on PREEMPT_RT, one example is:
rcu_core() contains an local_irq_{save,restore}() section.
/* No grace period and unregistered callbacks? */ if (!rcu_gp_in_progress() && rcu_segcblist_is_enabled(&rdp->cblist) && !offloaded) { local_irq_save(flags); if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL)) rcu_accelerate_cbs_unlocked(rnp, rdp); local_irq_restore(flags); }
And rcu_accelerate_cbs_unlocked(rnp, rdp) calls rcu_gp_kthread_wake() which does an swake_up_one(), so I think we will have to leave it as swake_up() the way it is.
thanks,
- Joel
| |