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SubjectRe: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation
On Mon, Dec 13, 2021 at 02:25:30PM +0530, Neeraj Upadhyay wrote:
> 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@quicinc.com>
> > > ---
> > > 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@amazon.co.uk>
> > + 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.

I was about to ack the patch but, should we really add code that isn't going to
be necessary before a long while?

Thanks!


>
> Thanks
> Neeraj
>
> >

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