rcu: fine-tune grace-period begin/end checks

Use the CPU's bit in rnp->qsmask to determine whether or not the CPU
should try to report a quiescent state.  Handle overflow in the check
for rdp->gpnum having fallen behind.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 18 insertions, 9 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 8105271fc10e..c39ec5b4ae82 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -617,9 +617,17 @@ static void __init check_cpu_stall_init(void)
 static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
 {
         if (rdp->gpnum != rnp->gpnum) {
-                rdp->qs_pending = 1;
-                rdp->passed_quiesc = 0;
+                /*
+                 * If the current grace period is waiting for this CPU,
+                 * set up to detect a quiescent state, otherwise don't
+                 * go looking for one.
+                 */
                 rdp->gpnum = rnp->gpnum;
+                if (rnp->qsmask & rdp->grpmask) {
+                        rdp->qs_pending = 1;
+                        rdp->passed_quiesc = 0;
+                } else
+                        rdp->qs_pending = 0;
         }
 }
 
@@ -681,19 +689,20 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
 
                 /*
                  * If we were in an extended quiescent state, we may have
-                 * missed some grace periods that others CPUs took care on
+                 * missed some grace periods that others CPUs handled on
                  * our behalf. Catch up with this state to avoid noting
-                 * spurious new grace periods.
+                 * spurious new grace periods.  If another grace period
+                 * has started, then rnp->gpnum will have advanced, so
+                 * we will detect this later on.
                  */
-                if (rdp->completed > rdp->gpnum)
+                if (ULONG_CMP_LT(rdp->gpnum, rdp->completed))
                         rdp->gpnum = rdp->completed;
 
                 /*
-                 * If another CPU handled our extended quiescent states and
-                 * we have no more grace period to complete yet, then stop
-                 * chasing quiescent states.
+                 * If RCU does not need a quiescent state from this CPU,
+                 * then make sure that this CPU doesn't go looking for one.
                  */
-                if (rdp->completed == rnp->gpnum)
+                if ((rnp->qsmask & rdp->grpmask) == 0)
                         rdp->qs_pending = 0;
         }
 }