linux-kernel - Re: [PATCH] srcu: Add detection of boot CPU srcu_data structure's->srcu

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Message-ID: <20221117142025.GE839309@lothringen>
Date:   Thu, 17 Nov 2022 15:20:25 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     paulmck@...nel.org, joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] srcu: Add detection of boot CPU srcu_data
 structure's->srcu_cblist

On Thu, Nov 17, 2022 at 07:20:50PM +0800, Zqiang wrote:
> Before SRCU_SIZE_WAIT_CALL srcu_size_state is reached, the srcu
> callback only insert to boot-CPU srcu_data structure's->srcu_cblist
> and other CPU srcu_data structure's->srcu_cblist is always empty. so
> before SRCU_SIZE_WAIT_CALL is reached, need to correctly check boot-CPU
> pend cbs in srcu_might_be_idle().
> 
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> ---
>  kernel/rcu/srcutree.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 6af031200580..6d9af9901765 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1098,8 +1098,11 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
>  	unsigned long tlast;
>  
>  	check_init_srcu_struct(ssp);
> -	/* If the local srcu_data structure has callbacks, not idle.  */
> -	sdp = raw_cpu_ptr(ssp->sda);
> +	/* If the boot CPU or local srcu_data structure has callbacks, not idle.  */
> +	if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_CALL)
> +		sdp = per_cpu_ptr(ssp->sda, get_boot_cpu_id());
> +	else
> +		sdp = raw_cpu_ptr(ssp->sda);

While at it if someone is interested in documenting/commenting on the meaning of
all these SRCU_SIZE_* things, it would be much appreciated!

Thanks.

>  	spin_lock_irqsave_rcu_node(sdp, flags);
>  	if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
>  		spin_unlock_irqrestore_rcu_node(sdp, flags);
> -- 
> 2.25.1
>