linux-kernel - Re: [EXTERNAL] [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation

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Message-ID: <6c184b9ffc5c641736d53bb7598f814d6b4c3fe0.camel@infradead.org>
Date:   Mon, 13 Dec 2021 08:18:11 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Neeraj Upadhyay <quic_neeraju@...cinc.com>, paulmck@...nel.org,
        frederic@...nel.org, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        urezki@...il.com, boqun.feng@...il.com
Subject: Re: [EXTERNAL] [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads
 creation

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@...cinc.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?

Acked-by: David Woodhouse <dwmw@...zon.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.

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