On Mon, Dec 13, 2021 at 02:14:07PM +0100, Frederic Weisbecker wrote: > On Mon, Dec 13, 2021 at 11:28:45AM +0000, David Woodhouse wrote: > > On Mon, 2021-12-13 at 12:22 +0100, Frederic Weisbecker wrote: > > > I was about to ack the patch but, should we really add code that isn't going to > > > be necessary before a long while? > > > > Yeah, I'm torn on that. In this case it's harmless enough and it makes > > the code reentrant in its own right instead of relying on the fact that > > the cpuhp code won't invoke it multiple times in parallel. So I think > > that's reasonable defensive programming. > > The thing is that RCU code is already quite complicated. Are we even at least > sure that we'll ever make CPU hotplug allow concurrent CPU onlining/offlining? > > This will require much more thoughts and a new hotplug concurrency > infrastructure that we'll need to base RCU on. IMHO it's a bit early to handle > that on hotplug individual callbacks. > > But anyway, let's see what Paul thinks about it... We need to at least think through parallelizing the various RCU CPU-hotplug notifiers. For one thing, it might turn out to be necessary to parallelize those notifiers, perhaps on some non-x86 architecture or on some non-Amazon .config. For another thing, doing so might suggest some simplifications, as has happened in my ongoing rcu_barrier() work. My thought is to pull in the patches and at the very least leave a tag recording them for later possible use. Thanx, Paul
