rcu: merge TREE_PREEPT_RCU blocked_tasks[] lists

Combine the current TREE_PREEMPT_RCU ->blocked_tasks[] lists in the
rcu_node structure into a single ->blkd_tasks list with ->gp_tasks
and ->exp_tasks tail pointers.  This is in preparation for RCU priority
boosting, which will add a third dimension to the combinatorial explosion
in the ->blocked_tasks[] case, but simply a third pointer in the new
->blkd_tasks case.

Also update documentation to reflect blocked_tasks[] merge

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 15 insertions, 6 deletions

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index bd891def3303..5a439c180e69 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -107,7 +107,7 @@ struct rcu_node {
                                 /*  an rcu_data structure, otherwise, each */
                                 /*  bit corresponds to a child rcu_node */
                                 /*  structure. */
-        unsigned long expmask;  /* Groups that have ->blocked_tasks[] */
+        unsigned long expmask;  /* Groups that have ->blkd_tasks */
                                 /*  elements that need to drain to allow the */
                                 /*  current expedited grace period to */
                                 /*  complete (only for TREE_PREEMPT_RCU). */
@@ -120,11 +120,20 @@ struct rcu_node {
         u8      grpnum;         /* CPU/group number for next level up. */
         u8      level;          /* root is at level 0. */
         struct rcu_node *parent;
-        struct list_head blocked_tasks[4];
-                                /* Tasks blocked in RCU read-side critsect. */
-                                /*  Grace period number (->gpnum) x blocked */
-                                /*  by tasks on the (x & 0x1) element of the */
-                                /*  blocked_tasks[] array. */
+        struct list_head blkd_tasks;
+                                /* Tasks blocked in RCU read-side critical */
+                                /*  section.  Tasks are placed at the head */
+                                /*  of this list and age towards the tail. */
+        struct list_head *gp_tasks;
+                                /* Pointer to the first task blocking the */
+                                /*  current grace period, or NULL if there */
+                                /*  is no such task. */
+        struct list_head *exp_tasks;
+                                /* Pointer to the first task blocking the */
+                                /*  current expedited grace period, or NULL */
+                                /*  if there is no such task.  If there */
+                                /*  is no current expedited grace period, */
+                                /*  then there can cannot be any such task. */
 } ____cacheline_internodealigned_in_smp;
 
 /*