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Subject: Re: [PATCH] rcu: tree_stall: Correctly unlock root node in
 rcu_check_gp_start_stall
To:     Neeraj Upadhyay <neeraju@codeaurora.org>,
        paulmck@linux.vnet.ibm.com, josh@joshtriplett.org,
        rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
        jiangshanlai@gmail.com, joel@joelfernandes.org
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1553858828-23097-1-git-send-email-neeraju@codeaurora.org>
From:   Mukesh Ojha <mojha@codeaurora.org>
Message-ID: <7a9eed5a-9bcf-460c-abb9-1e9bd6eae102@codeaurora.org>
Date:   Fri, 29 Mar 2019 18:58:22 +0530
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On 3/29/2019 4:57 PM, Neeraj Upadhyay wrote:
> Only unlock the root node, if current node (rnp) is not
> root node.
>
> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>


> ---
>   kernel/rcu/tree_stall.h | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index f65a73a..0651833 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h


why this is showing as under tree_stall.h while it is under 
"kernel/rcu/tree.c"

> @@ -630,7 +630,9 @@ static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
>   	    time_before(j, rcu_state.gp_req_activity + gpssdelay) ||
>   	    time_before(j, rcu_state.gp_activity + gpssdelay) ||
>   	    atomic_xchg(&warned, 1)) {
> -		raw_spin_unlock_rcu_node(rnp_root); /* irqs remain disabled. */
> +		if (rnp_root != rnp)
> +			/* irqs remain disabled. */
> +			raw_spin_unlock_rcu_node(rnp_root);

Looks good as it will balance the lock .if it is the root_node, which 
was not there earlier, and unlock was happening without any lock on root.

Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>

Cheers,
-Mukesh

>   		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>   		return;
>   	}