On Wed, May 20, 2020 at 08:28:00PM +0200, Sebastian Andrzej Siewior wrote: > On 2020-05-20 13:42:59 [-0400], Joel Fernandes wrote: > > Hi Sebastian, > Hi Joel, > > > For pointer stability, can we just use get_local_ptr() and put_local_ptr() > > instead of adding an extra lock? This keeps the pointer stable while keeping > > the section preemptible on -rt. And we already have a lock in rcu_data, I > > prefer not to add another lock if possible. > > What is this get_local_ptr() doing? I can't find it anywhere… I replied about it in the other thread. > > > I remember Paul looked at that patch a few years ago and he said that > > > that disabling interrupts here is important and matches the other part > > > instance where the interrupts are disabled. Looking at it now, it seems > > > that there is just pointer stability but I can't tell if > > > rcu_segcblist_pend_cbs() needs more than just this. > > > > Which 'other part' are you referring to? Your patch removed local_irq_save() > > from other places as well right? > > The patch converted hunks. > So then there are no other local_irq_save() to match with. Or may be I did not understand your concern, could you share any threads from past discussions about disabling interrupts in this code? You mentioned about a discussion from few years ago. > > > > - Joel > > > > ---8<----------------------- > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > index 8ff71e5d0fe8b..5f49919205317 100644 > > --- a/kernel/rcu/srcutree.c > > +++ b/kernel/rcu/srcutree.c > > @@ -778,13 +778,17 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp) > > unsigned long tlast; > > > > /* If the local srcu_data structure has callbacks, not idle. */ > > - local_irq_save(flags); > > - sdp = this_cpu_ptr(ssp->sda); > > + sdp = get_local_ptr(ssp->sda); > > + spin_lock_irqsave_rcu_node(sdp, flags); > > You acquire the node lock which was not acquired before. Is that okay? > How is get_local_ptr() different to raw_cpu_ptr()? get_cpu_ptr() disables preemption which you might not want, right? Most (all?) other paths are accessing the cblist under lock so I added it here to be safe. This is anyway called from a slowpath. Do you see a problem? thanks, - Joel
