Re: [PATCH RFC tip/core/rcu 3/5] srcu: Provide internal interface to start a Tree SRCU grace period

[Neeraj Upadhyay <neeraju@codeaurora.org>]

On 11/17/2020 6:10 AM, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> There is a need for a polling interface for SRCU grace periods.
> This polling needs to initiate an SRCU grace period without having
> to queue (and manage) a callback.  This commit therefore splits the
> Tree SRCU __call_srcu() function into callback-initialization and
> queuing/start-grace-period portions, with the latter in a new function
> named srcu_gp_start_if_needed().  This function may be passed a NULL
> callback pointer, in which case it will refrain from queuing anything.
> 
> Why have the new function mess with queuing?  Locking considerations,
> of course!
> 
> Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
> Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---

Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>


Thanks
Neeraj

>   kernel/rcu/srcutree.c | 66 +++++++++++++++++++++++++++++----------------------
>   1 file changed, 37 insertions(+), 29 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 79b7081..d930ece 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -808,6 +808,42 @@ static void srcu_leak_callback(struct rcu_head *rhp)
>   }
>   
>   /*
> + * Start an SRCU grace period, and also queue the callback if non-NULL.
> + */
> +static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rhp, bool do_norm)
> +{
> +	unsigned long flags;
> +	int idx;
> +	bool needexp = false;
> +	bool needgp = false;
> +	unsigned long s;
> +	struct srcu_data *sdp;
> +
> +	idx = srcu_read_lock(ssp);
> +	sdp = raw_cpu_ptr(ssp->sda);
> +	spin_lock_irqsave_rcu_node(sdp, flags);
> +	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> +	rcu_segcblist_advance(&sdp->srcu_cblist,
> +			      rcu_seq_current(&ssp->srcu_gp_seq));
> +	s = rcu_seq_snap(&ssp->srcu_gp_seq);
> +	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
> +	if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
> +		sdp->srcu_gp_seq_needed = s;
> +		needgp = true;
> +	}
> +	if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
> +		sdp->srcu_gp_seq_needed_exp = s;
> +		needexp = true;
> +	}
> +	spin_unlock_irqrestore_rcu_node(sdp, flags);
> +	if (needgp)
> +		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
> +	else if (needexp)
> +		srcu_funnel_exp_start(ssp, sdp->mynode, s);
> +	srcu_read_unlock(ssp, idx);
> +}
> +
> +/*
>    * Enqueue an SRCU callback on the srcu_data structure associated with
>    * the current CPU and the specified srcu_struct structure, initiating
>    * grace-period processing if it is not already running.
> @@ -838,13 +874,6 @@ static void srcu_leak_callback(struct rcu_head *rhp)
>   static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
>   			rcu_callback_t func, bool do_norm)
>   {
> -	unsigned long flags;
> -	int idx;
> -	bool needexp = false;
> -	bool needgp = false;
> -	unsigned long s;
> -	struct srcu_data *sdp;
> -
>   	check_init_srcu_struct(ssp);
>   	if (debug_rcu_head_queue(rhp)) {
>   		/* Probable double call_srcu(), so leak the callback. */
> @@ -853,28 +882,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
>   		return;
>   	}
>   	rhp->func = func;
> -	idx = srcu_read_lock(ssp);
> -	sdp = raw_cpu_ptr(ssp->sda);
> -	spin_lock_irqsave_rcu_node(sdp, flags);
> -	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> -	rcu_segcblist_advance(&sdp->srcu_cblist,
> -			      rcu_seq_current(&ssp->srcu_gp_seq));
> -	s = rcu_seq_snap(&ssp->srcu_gp_seq);
> -	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
> -	if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
> -		sdp->srcu_gp_seq_needed = s;
> -		needgp = true;
> -	}
> -	if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
> -		sdp->srcu_gp_seq_needed_exp = s;
> -		needexp = true;
> -	}
> -	spin_unlock_irqrestore_rcu_node(sdp, flags);
> -	if (needgp)
> -		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
> -	else if (needexp)
> -		srcu_funnel_exp_start(ssp, sdp->mynode, s);
> -	srcu_read_unlock(ssp, idx);
> +	srcu_gp_start_if_needed(ssp, rhp, do_norm);
>   }
>   
>   /**
> 

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