rcu: Use better variable names in funnel locking loop

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 (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix name in comment as well. ]

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8c31b1740afc4d8d5e392f333aae4816f927ed42..1af58f4b8a252e6f8569d00bc28c0cf27591257c 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1554,7 +1554,7 @@ 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.
  *
@@ -1568,74 +1568,74 @@ 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
         * structure's lock or bail out if the need for this grace period
-        * has already been recorded -- or has already started.  If there
-        * is already a grace period in progress in a non-leaf node, no
-        * recording is needed because the end of the grace period will
-        * scan the leaf rcu_node structures.  Note that rnp->lock must
-        * not be released.
+        * has already been recorded -- or if that grace period has in
+        * fact already started.  If there is already a grace period in
+        * progress in a non-leaf node, no recording is needed because the
+        * end of the grace period will scan the leaf rcu_node structures.
+        * Note that rnp_start->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_LT(gp_seq_req, rnp_root->gp_seq_needed)) {
-               rnp->gp_seq_needed = rnp_root->gp_seq_needed;
-               rdp->gp_seq_needed = rnp_root->gp_seq_needed;
+       if (ULONG_CMP_LT(gp_seq_req, rnp->gp_seq_needed)) {
+               rnp_start->gp_seq_needed = rnp->gp_seq_needed;
+               rdp->gp_seq_needed = rnp->gp_seq_needed;
        }
-       if (rnp != rnp_root)
-               raw_spin_unlock_rcu_node(rnp_root);
+       if (rnp != rnp_start)
+               raw_spin_unlock_rcu_node(rnp);
        return ret;
 }