Re: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation

[Neeraj Upadhyay]

Hi David,

Thanks for the review; some replies inline.

On 12/13/2021 1:48 PM, David Woodhouse wrote:
> On Sat, 2021-12-11 at 22:31 +0530, Neeraj Upadhyay wrote:
> > When multiple CPUs in the same nocb gp/cb group concurrently
> > come online, they might try to concurrently create the same
> > rcuog kthread. Fix this by using nocb gp CPU's spawn mutex to
> > provide mutual exclusion for the rcuog kthread creation code.
> >
> > Signed-off-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> > ---
> > Change in v2:
> >   Fix missing mutex_unlock in nocb gp kthread creation err path.
>
> I think this ends up being not strictly necessary in the short term too
> because we aren't currently planning to run rcutree_prepare_cpu()
> concurrently anyway. But harmless and worth fixing in the longer term.
>
> Although, if I've already added a mutex for adding the boost thread,
> could we manage to use the *same* mutex instead of adding another one?

Let me think about it; the nocb-gp and nocb-cb kthreads are grouped 
based on rcu_nocb_gp_stride; whereas, boost kthreads are per rnp. So, I 
need to see how we can use a common mutex for both.


> Acked-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> +                       mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
> >                          return;
> > +               }
> >                  WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
> >          }
> > +        mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
> >
> >          /* Spawn the kthread for this CPU. */
>
> Some whitespace damage there.

Will fix in next version.

Thanks
Neeraj

>