On Sun, May 13, 2018 at 08:15:38PM -0700, Joel Fernandes (Google) wrote:
> 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 rcu_node, and then finally when
> rcu_node is the rcu_root, lets assign it at that time.
>
> Just a clean up patch, no logical change.
>
> Signed-off-by: Joel Fernandes (Google) <[email protected]>
I agree that my names could use some improvement, but given that you
called it rnp_node in the patch and rcu_node in the commit log, I would
argue that rnp_node has a Hamming-distance problem. ;-)
How about rnp_start for the formal parameter, rnp for the cursor running
up the tree, and retaining rnp_root for the root?
I considered and rejected the thought of rnp_leaf for the formal
parameter because this function is not necessarily called with a leaf
rcu_node structure.
Thanx, Paul
> ---
> kernel/rcu/tree.c | 34 ++++++++++++++++++----------------
> 1 file changed, 18 insertions(+), 16 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 9f5679ba413b..40670047d22c 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1568,7 +1568,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp,
> struct rcu_data *rdp,
> {
> bool ret = false;
> struct rcu_state *rsp = rdp->rsp;
> - struct rcu_node *rnp_root;
> + struct rcu_node *rnp_node, *rnp_root = NULL;
>
> /*
> * Use funnel locking to either acquire the root rcu_node
> @@ -1581,24 +1581,26 @@ static bool rcu_start_this_gp(struct rcu_node *rnp,
> struct rcu_data *rdp,
> */
> raw_lockdep_assert_held_rcu_node(rnp);
> trace_rcu_this_gp(rnp, rdp, gp_seq_start, 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_start) ||
> - rcu_seq_done(&rnp_root->gp_seq, gp_seq_start) ||
> - (rnp != rnp_root &&
> - rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
> - trace_rcu_this_gp(rnp_root, rdp, gp_seq_start,
> + for (rnp_node = rnp; 1; rnp_node = rnp_node->parent) {
> + if (rnp_node != rnp)
> + raw_spin_lock_rcu_node(rnp_node);
> + if (ULONG_CMP_GE(rnp_node->gp_seq_needed, gp_seq_start) ||
> + rcu_seq_done(&rnp_node->gp_seq, gp_seq_start) ||
> + (rnp != rnp_node &&
> + rcu_seq_state(rcu_seq_current(&rnp_node->gp_seq)))) {
> + trace_rcu_this_gp(rnp_node, rdp, gp_seq_start,
> TPS("Prestarted"));
> goto unlock_out;
> }
> - rnp_root->gp_seq_needed = gp_seq_start;
> + rnp_node->gp_seq_needed = gp_seq_start;
> if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
> goto unlock_out;
> - if (rnp_root != rnp && rnp_root->parent != NULL)
> - raw_spin_unlock_rcu_node(rnp_root);
> - if (!rnp_root->parent)
> + if (rnp_node != rnp && rnp_node->parent != NULL)
> + raw_spin_unlock_rcu_node(rnp_node);
> + if (!rnp_node->parent) {
> + rnp_root = rnp_node;
> break; /* At root, and perhaps also leaf. */
> + }
> }
>
> /* If GP already in progress, just leave, otherwise start one. */
> @@ -1616,10 +1618,10 @@ static bool rcu_start_this_gp(struct rcu_node *rnp,
> struct rcu_data *rdp,
> trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq),
> TPS("newreq"));
> ret = true; /* Caller must wake GP kthread. */
> unlock_out:
> - if (rnp != rnp_root)
> - raw_spin_unlock_rcu_node(rnp_root);
> + if (rnp != rnp_node)
> + raw_spin_unlock_rcu_node(rnp_node);
> /* Push furthest requested GP to leaf node and rcu_data structure. */
> - if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_start)) {
> + if (ULONG_CMP_GE(rnp_node->gp_seq_needed, gp_seq_start)) {
> rnp->gp_seq_needed = gp_seq_start;
> rdp->gp_seq_needed = gp_seq_start;
> }
> --
> 2.17.0.441.gb46fe60e1d-goog
>
