On Thu, Oct 14, 2021 at 12:42:40PM +0100, Valentin Schneider wrote: > On 14/10/21 00:07, Boqun Feng wrote: > >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > >> index e38028d48648..b236271b9022 100644 > >> --- a/kernel/rcu/tree.c > >> +++ b/kernel/rcu/tree.c > >> @@ -2717,6 +2717,23 @@ static __latent_entropy void rcu_core(void) > >> unsigned long flags; > >> struct rcu_data *rdp = raw_cpu_ptr(&rcu_data); > >> struct rcu_node *rnp = rdp->mynode; > >> + /* > >> + * On RT rcu_core() can be preempted when IRQs aren't disabled. > >> + * Therefore this function can race with concurrent NOCB (de-)offloading > >> + * on this CPU and the below condition must be considered volatile. > >> + * However if we race with: > >> + * > >> + * _ Offloading: In the worst case we accelerate or process callbacks > >> + * concurrently with NOCB kthreads. We are guaranteed to > >> + * call rcu_nocb_lock() if that happens. > > > > If offloading races with rcu_core(), can the following happen? > > > > <offload work> > > rcu_nocb_rdp_offload(): > > rcu_core(): > > ... > > rcu_nocb_lock_irqsave(); // no a lock > > raw_spin_lock_irqsave(->nocb_lock); > > rdp_offload_toggle(): > > <LOCKING | OFFLOADED set> > > if (!rcu_segcblist_restempty(...)) > > rcu_accelerate_cbs_unlocked(...); > > rcu_nocb_unlock_irqrestore(); > > // ^ a real unlock, > > // and will preempt_enable() > > // offload continue with ->nocb_lock not held > > > > If this can happen, it means an unpaired preempt_enable() and an > > incorrect unlock. Thoughts? Maybe I'm missing something here? > > > > As Frederic pointed out in an earlier thread [1], this can't happen because > we still have IRQs disabled, and the offloading process has to be processed > on the CPU being offloaded. IOW, in the above scenario, rcu_core() can't be > preempted by the rcu_nocb_rdp_offload() work until it re-enables IRQs at > rcu_nocb_unlock_irqrestore(). > > (hopefully Paul or Frederic will correct me if I've just spewed garbage) > > [1]: https://lore.kernel.org/lkml/20210930105340.GA232241@lothringen/ > Thanks! I think Frederic and you are right: this cannot happen. Thank you both for looking into this ;-) Regards, Boqun > > Regards, > > Boqun > >
