rcu: Clean up code based on review feedback from Josh Triplett, part 2

These issues identified during an old-fashioned face-to-face code
review extending over many hours.

o	Add comments for tricky parts of code, and correct comments
	that have passed their sell-by date.

o	Get rid of the vestiges of rcu_init_sched(), which is no
	longer needed now that PREEMPT_RCU is gone.

o	Move the #include of rcutree_plugin.h to the end of
	rcutree.c, which means that, rather than having a random
	collection of forward declarations, the new set of forward
	declarations document the set of plugins.  The new home for
	this #include also allows __rcu_init_preempt() to move into
	rcutree_plugin.h.

o	Fix rcu_preempt_check_callbacks() to be static.

Suggested-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <12537246443924-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Peter Zijlstra <peterz@infradead.org>

1 files changed, 23 insertions, 8 deletions

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 9aa8c8a160d8..a48d11f37b4c 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -79,15 +79,21 @@ struct rcu_dynticks {
  * Definition for node within the RCU grace-period-detection hierarchy.
  */
 struct rcu_node {
-        spinlock_t lock;
+        spinlock_t lock;        /* Root rcu_node's lock protects some */
+                                /*  rcu_state fields as well as following. */
         long    gpnum;          /* Current grace period for this node. */
                                 /*  This will either be equal to or one */
                                 /*  behind the root rcu_node's gpnum. */
         unsigned long qsmask;   /* CPUs or groups that need to switch in */
                                 /*  order for current grace period to proceed.*/
+                                /*  In leaf rcu_node, each bit corresponds to */
+                                /*  an rcu_data structure, otherwise, each */
+                                /*  bit corresponds to a child rcu_node */
+                                /*  structure. */
         unsigned long qsmaskinit;
                                 /* Per-GP initialization for qsmask. */
         unsigned long grpmask;  /* Mask to apply to parent qsmask. */
+                                /*  Only one bit will be set in this mask. */
         int     grplo;          /* lowest-numbered CPU or group here. */
         int     grphi;          /* highest-numbered CPU or group here. */
         u8      grpnum;         /* CPU/group number for next level up. */
@@ -95,6 +101,9 @@ struct rcu_node {
         struct rcu_node *parent;
         struct list_head blocked_tasks[2];
                                 /* 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. */
 } ____cacheline_internodealigned_in_smp;
 
 /* Index values for nxttail array in struct rcu_data. */
@@ -126,19 +135,22 @@ struct rcu_data {
          * Any of the partitions might be empty, in which case the
          * pointer to that partition will be equal to the pointer for
          * the following partition.  When the list is empty, all of
-         * the nxttail elements point to nxtlist, which is NULL.
+         * the nxttail elements point to the ->nxtlist pointer itself,
+         * which in that case is NULL.
          *
-         * [*nxttail[RCU_NEXT_READY_TAIL], NULL = *nxttail[RCU_NEXT_TAIL]):
-         *      Entries that might have arrived after current GP ended
-         * [*nxttail[RCU_WAIT_TAIL], *nxttail[RCU_NEXT_READY_TAIL]):
-         *      Entries known to have arrived before current GP ended
-         * [*nxttail[RCU_DONE_TAIL], *nxttail[RCU_WAIT_TAIL]):
-         *      Entries that batch # <= ->completed - 1: waiting for current GP
          * [nxtlist, *nxttail[RCU_DONE_TAIL]):
          *      Entries that batch # <= ->completed
          *      The grace period for these entries has completed, and
          *      the other grace-period-completed entries may be moved
          *      here temporarily in rcu_process_callbacks().
+         * [*nxttail[RCU_DONE_TAIL], *nxttail[RCU_WAIT_TAIL]):
+         *      Entries that batch # <= ->completed - 1: waiting for current GP
+         * [*nxttail[RCU_WAIT_TAIL], *nxttail[RCU_NEXT_READY_TAIL]):
+         *      Entries known to have arrived before current GP ended
+         * [*nxttail[RCU_NEXT_READY_TAIL], *nxttail[RCU_NEXT_TAIL]):
+         *      Entries that might have arrived after current GP ended
+         *      Note that the value of *nxttail[RCU_NEXT_TAIL] will
+         *      always be NULL, as this is the end of the list.
          */
         struct rcu_head *nxtlist;
         struct rcu_head **nxttail[RCU_NEXT_SIZE];
@@ -216,6 +228,9 @@ struct rcu_state {
                                                 /* Force QS state. */
         long    gpnum;                          /* Current gp number. */
         long    completed;                      /* # of last completed gp. */
+
+        /* End  of fields guarded by root rcu_node's lock. */
+
         spinlock_t onofflock;                   /* exclude on/offline and */
                                                 /*  starting new GP. */
         spinlock_t fqslock;                     /* Only one task forcing */