linux-kernel - Re: [PATCH] rcu/exp: Mark current CPU as exp-QS in IPI loop second pass

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Message-ID: <dee700be-bad1-e497-ccfc-916b98d7d593@quicinc.com>
Date:   Tue, 30 Nov 2021 22:31:57 +0530
From:   Neeraj Upadhyay <quic_neeraju@...cinc.com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, <rcu@...r.kernel.org>
Subject: Re: [PATCH] rcu/exp: Mark current CPU as exp-QS in IPI loop second
 pass



On 11/30/2021 9:51 PM, Frederic Weisbecker wrote:
> While looping through the rnp's CPUs to IPI for an expedited grace
> period, a first pass excludes the current CPU and the CPUs in dynticks
> idle mode. The workqueue will report their QS on their behalf later.
> 
> The second pass processes the IPIs and also ignores the current CPU,
> assuming it has been previously included in the group of CPUs whose
> QS are to be reported by the workqueue.
> 
> Unfortunately the current CPU may have changed between the first and
> second pass, due to the rnp lock being dropped, re-enabling preemption.
> As a result the current CPU, if different in the second pass, may be
> ignored by the expedited grace period. No IPI will be sent to it
> so it won't be requested to report an expedited quiescent state.
> 
> This ends up in an expedited grace period stall.
> 
> Fix this with including the current CPU in the second round in the group
> of CPUs to report a QS for by the workqueue.
> 
> Fixes: b9ad4d6ed18e ("rcu: Avoid self-IPI in sync_rcu_exp_select_node_cpus()")
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Uladzislau Rezki <urezki@...il.com>
> Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> ---

Reviewed-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>



Thanks
Neeraj


>   kernel/rcu/tree_exp.h | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index a96d17206d87..237a79989aba 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -387,6 +387,7 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
>   			continue;
>   		}
>   		if (get_cpu() == cpu) {
> +			mask_ofl_test |= mask;
>   			put_cpu();
>   			continue;
>   		}
>