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

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Message-ID: <601ecb12-ae2e-9608-7127-c2cddc8038a6@quicinc.com>
Date:   Mon, 13 Dec 2021 14:25:30 +0530
From:   Neeraj Upadhyay <quic_neeraju@...cinc.com>
To:     David Woodhouse <dwmw2@...radead.org>, <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: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation

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@...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?
> 

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@...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.

Will fix in next version.

Thanks
Neeraj

>