Re: [PATCH] rcu: Make rcu_state.n_online_cpus updates atomic

[Frederic Weisbecker]

On Mon, Dec 13, 2021 at 11:32:56AM -0800, Paul E. McKenney wrote:
> On Mon, Dec 13, 2021 at 08:09:22AM +0000, Woodhouse, David wrote:
> > On Mon, 2021-12-13 at 12:30 +0530, Neeraj Upadhyay wrote:
> > > To support onlining multiple CPUs concurrently,
> > > change rcu_state.n_online_cpus updates to be atomic.
> > > Note, it's ok for rcu_blocking_is_gp() to do a
> > > atomic_read(&rcu_state.n_online_cpus), as the
> > > value of .n_online_cpus switches from 1->2, in
> > > rcutree_prepare_cpu(), which runs before the new
> > > CPU comes online. Similarly 2->1 transition happens
> > > from rcutree_dead_cpu(), which executes after the
> > > CPU is offlined, and runs on the last online CPU.
> > > 
> > > Signed-off-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> > 
> > In my parallel-bringup series, the prepare stages are still being
> > executed in series on the BSP, so I don't think this patch is needed
> > yet. I'm not sure we'd ever end up with the prepare stages being done
> > in parallel — the most I see us doing is onlining a single *batch* of
> > CPUs at a time, much like bringup_nonboot_cpus() does.
> > 
> > But this patch certainly doesn't *hurt*.
> > 
> > Acked-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> 
> Queued for further review and testing.
> 
> To Frederic's point, this won't go to mainline unless it is actually
> needed, but it will at least be pulled into a branch in -rcu marked with
> a tag for future reference.

Ok, sounds reasonable then.

Thanks!