Re: [PATCH 3/4] rcu: Use better variable names in funnel locking loop

[Paul E. McKenney]

On Tue, May 22, 2018 at 11:38:14PM -0700, Joel Fernandes wrote:
> From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
> 
> The funnel locking loop in rcu_start_this_gp uses rcu_root as a
> temporary variable while walking the combining tree. This causes a
> tiresome exercise of a code reader reminding themselves that rcu_root
> may not be root. Lets just call it rnp, and rename other variables as
> well to be more appropriate.
> 
> Original patch: https://patchwork.kernel.org/patch/10396577/
> 
> Signed-off-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

I used to have double Signed-off-by back when I was seconded to Linaro.
But I am guessing that you want the second and don't need the first
one.  Unless you tell me otherwise, I will remove the first one on
my next rebase.

Anyway, the new variable names are much more clear, good stuff,
queued for further review and testing, thank you!

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 52 +++++++++++++++++++++++------------------------
>  1 file changed, 26 insertions(+), 26 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 0ad61c97da69..31f4b4b7d824 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1526,7 +1526,7 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
> 
>  /*
>   * rcu_start_this_gp - Request the start of a particular grace period
> - * @rnp: The leaf node of the CPU from which to start.
> + * @rnp_start: The leaf node of the CPU from which to start.
>   * @rdp: The rcu_data corresponding to the CPU from which to start.
>   * @gp_seq_req: The gp_seq of the grace period to start.
>   *
> @@ -1540,12 +1540,12 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
>   *
>   * Returns true if the GP thread needs to be awakened else false.
>   */
> -static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
> +static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
>  			      unsigned long gp_seq_req)
>  {
>  	bool ret = false;
>  	struct rcu_state *rsp = rdp->rsp;
> -	struct rcu_node *rnp_root;
> +	struct rcu_node *rnp;
> 
>  	/*
>  	 * Use funnel locking to either acquire the root rcu_node
> @@ -1556,58 +1556,58 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
>  	 * scan the leaf rcu_node structures.  Note that rnp->lock must
>  	 * not be released.
>  	 */
> -	raw_lockdep_assert_held_rcu_node(rnp);
> -	trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startleaf"));
> -	for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
> -		if (rnp_root != rnp)
> -			raw_spin_lock_rcu_node(rnp_root);
> -		if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_req) ||
> -		    rcu_seq_started(&rnp_root->gp_seq, gp_seq_req) ||
> -		    (rnp != rnp_root &&
> -		     rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
> -			trace_rcu_this_gp(rnp_root, rdp, gp_seq_req,
> +	raw_lockdep_assert_held_rcu_node(rnp_start);
> +	trace_rcu_this_gp(rnp_start, rdp, gp_seq_req, TPS("Startleaf"));
> +	for (rnp = rnp_start; 1; rnp = rnp->parent) {
> +		if (rnp != rnp_start)
> +			raw_spin_lock_rcu_node(rnp);
> +		if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req) ||
> +		    rcu_seq_started(&rnp->gp_seq, gp_seq_req) ||
> +		    (rnp != rnp_start &&
> +		     rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))) {
> +			trace_rcu_this_gp(rnp, rdp, gp_seq_req,
>  					  TPS("Prestarted"));
>  			goto unlock_out;
>  		}
> -		rnp_root->gp_seq_needed = gp_seq_req;
> -		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
> +		rnp->gp_seq_needed = gp_seq_req;
> +		if (rcu_seq_state(rcu_seq_current(&rnp_start->gp_seq))) {
>  			/*
>  			 * We just marked the leaf, and a grace period
>  			 * is in progress, which means that rcu_gp_cleanup()
>  			 * will see the marking.  Bail to reduce contention.
>  			 */
> -			trace_rcu_this_gp(rnp, rdp, gp_seq_req,
> +			trace_rcu_this_gp(rnp_start, rdp, gp_seq_req,
>  					  TPS("Startedleaf"));
>  			goto unlock_out;
>  		}
> -		if (rnp_root != rnp && rnp_root->parent != NULL)
> -			raw_spin_unlock_rcu_node(rnp_root);
> -		if (!rnp_root->parent)
> +		if (rnp != rnp_start && rnp->parent != NULL)
> +			raw_spin_unlock_rcu_node(rnp);
> +		if (!rnp->parent)
>  			break;  /* At root, and perhaps also leaf. */
>  	}
> 
>  	/* If GP already in progress, just leave, otherwise start one. */
>  	if (rcu_gp_in_progress(rsp)) {
> -		trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("Startedleafroot"));
> +		trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedleafroot"));
>  		goto unlock_out;
>  	}
> -	trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("Startedroot"));
> +	trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot"));
>  	WRITE_ONCE(rsp->gp_flags, rsp->gp_flags | RCU_GP_FLAG_INIT);
>  	rsp->gp_req_activity = jiffies;
>  	if (!rsp->gp_kthread) {
> -		trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("NoGPkthread"));
> +		trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
>  		goto unlock_out;
>  	}
>  	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq"));
>  	ret = true;  /* Caller must wake GP kthread. */
>  unlock_out:
>  	/* Push furthest requested GP to leaf node and rcu_data structure. */
> -	if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_req)) {
> -		rnp->gp_seq_needed = gp_seq_req;
> +	if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req)) {
> +		rnp_start->gp_seq_needed = gp_seq_req;
>  		rdp->gp_seq_needed = gp_seq_req;
>  	}
> -	if (rnp != rnp_root)
> -		raw_spin_unlock_rcu_node(rnp_root);
> +	if (rnp != rnp_start)
> +		raw_spin_unlock_rcu_node(rnp);
>  	return ret;
>  }
> 
> -- 
> 2.17.0.441.gb46fe60e1d-goog
>