rcu: Simplify quiescent-state detection

The current quiescent-state detection algorithm is needlessly
complex.  It records the grace-period number corresponding to
the quiescent state at the time of the quiescent state, which
works, but it seems better to simply erase any record of previous
quiescent states at the time that the CPU notices the new grace
period.  This has the further advantage of removing another piece
of RCU for which lockless reasoning is required.

Therefore, this commit makes this change.

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>

4 files changed, 16 insertions, 27 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a2eadd04fb29..6194402ec853 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -176,8 +176,6 @@ void rcu_sched_qs(int cpu)
 {
         struct rcu_data *rdp = &per_cpu(rcu_sched_data, cpu);
 
-        rdp->passed_quiesce_gpnum = rdp->gpnum;
-        barrier();
         if (rdp->passed_quiesce == 0)
                 trace_rcu_grace_period("rcu_sched", rdp->gpnum, "cpuqs");
         rdp->passed_quiesce = 1;
@@ -187,8 +185,6 @@ void rcu_bh_qs(int cpu)
 {
         struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);
 
-        rdp->passed_quiesce_gpnum = rdp->gpnum;
-        barrier();
         if (rdp->passed_quiesce == 0)
                 trace_rcu_grace_period("rcu_bh", rdp->gpnum, "cpuqs");
         rdp->passed_quiesce = 1;
@@ -899,12 +895,8 @@ static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct
                  */
                 rdp->gpnum = rnp->gpnum;
                 trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpustart");
-                if (rnp->qsmask & rdp->grpmask) {
-                        rdp->qs_pending = 1;
-                        rdp->passed_quiesce = 0;
-                } else {
-                        rdp->qs_pending = 0;
-                }
+                rdp->passed_quiesce = 0;
+                rdp->qs_pending = !!(rnp->qsmask & rdp->grpmask);
                 zero_cpu_stall_ticks(rdp);
         }
 }
@@ -984,10 +976,13 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
                  * our behalf. Catch up with this state to avoid noting
                  * spurious new grace periods.  If another grace period
                  * has started, then rnp->gpnum will have advanced, so
-                 * we will detect this later on.
+                 * we will detect this later on.  Of course, any quiescent
+                 * states we found for the old GP are now invalid.
                  */
-                if (ULONG_CMP_LT(rdp->gpnum, rdp->completed))
+                if (ULONG_CMP_LT(rdp->gpnum, rdp->completed)) {
                         rdp->gpnum = rdp->completed;
+                        rdp->passed_quiesce = 0;
+                }
 
                 /*
                  * If RCU does not need a quiescent state from this CPU,
@@ -1358,7 +1353,7 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
  * based on quiescent states detected in an earlier grace period!
  */
 static void
-rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long lastgp)
+rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp)
 {
         unsigned long flags;
         unsigned long mask;
@@ -1366,7 +1361,8 @@ rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long las
 
         rnp = rdp->mynode;
         raw_spin_lock_irqsave(&rnp->lock, flags);
-        if (lastgp != rnp->gpnum || rnp->completed == rnp->gpnum) {
+        if (rdp->passed_quiesce == 0 || rdp->gpnum != rnp->gpnum ||
+            rnp->completed == rnp->gpnum) {
 
                 /*
                  * The grace period in which this quiescent state was
@@ -1425,7 +1421,7 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
          * Tell RCU we are done (but rcu_report_qs_rdp() will be the
          * judge of that).
          */
-        rcu_report_qs_rdp(rdp->cpu, rsp, rdp, rdp->passed_quiesce_gpnum);
+        rcu_report_qs_rdp(rdp->cpu, rsp, rdp);
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -2600,7 +2596,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
                         rdp->completed = rnp->completed;
                         rdp->passed_quiesce = 0;
                         rdp->qs_pending = 0;
-                        rdp->passed_quiesce_gpnum = rnp->gpnum - 1;
                         trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpuonl");
                 }
                 raw_spin_unlock(&rnp->lock); /* irqs already disabled. */
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 8f0293ce1517..935dd4ca6816 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -246,8 +246,6 @@ struct rcu_data {
                                         /*  in order to detect GP end. */
         unsigned long   gpnum;          /* Highest gp number that this CPU */
                                         /*  is aware of having started. */
-        unsigned long   passed_quiesce_gpnum;
-                                        /* gpnum at time of quiescent state. */
         bool            passed_quiesce; /* User-mode/idle loop etc. */
         bool            qs_pending;     /* Core waits for quiesc state. */
         bool            beenonline;     /* CPU online at least once. */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index b4e8eb24a5f1..4734afbea73a 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -137,8 +137,6 @@ static void rcu_preempt_qs(int cpu)
 {
         struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
 
-        rdp->passed_quiesce_gpnum = rdp->gpnum;
-        barrier();
         if (rdp->passed_quiesce == 0)
                 trace_rcu_grace_period("rcu_preempt", rdp->gpnum, "cpuqs");
         rdp->passed_quiesce = 1;
diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
index f54f0ceda0cf..bd4df13d4afb 100644
--- a/kernel/rcutree_trace.c
+++ b/kernel/rcutree_trace.c
@@ -86,12 +86,11 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
 {
         if (!rdp->beenonline)
                 return;
-        seq_printf(m, "%3d%cc=%lu g=%lu pq=%d pgp=%lu qp=%d",
+        seq_printf(m, "%3d%cc=%lu g=%lu pq=%d qp=%d",
                    rdp->cpu,
                    cpu_is_offline(rdp->cpu) ? '!' : ' ',
                    rdp->completed, rdp->gpnum,
-                   rdp->passed_quiesce, rdp->passed_quiesce_gpnum,
-                   rdp->qs_pending);
+                   rdp->passed_quiesce, rdp->qs_pending);
         seq_printf(m, " dt=%d/%llx/%d df=%lu",
                    atomic_read(&rdp->dynticks->dynticks),
                    rdp->dynticks->dynticks_nesting,
@@ -150,12 +149,11 @@ static void print_one_rcu_data_csv(struct seq_file *m, struct rcu_data *rdp)
 {
         if (!rdp->beenonline)
                 return;
-        seq_printf(m, "%d,%s,%lu,%lu,%d,%lu,%d",
+        seq_printf(m, "%d,%s,%lu,%lu,%d,%d",
                    rdp->cpu,
                    cpu_is_offline(rdp->cpu) ? "\"N\"" : "\"Y\"",
                    rdp->completed, rdp->gpnum,
-                   rdp->passed_quiesce, rdp->passed_quiesce_gpnum,
-                   rdp->qs_pending);
+                   rdp->passed_quiesce, rdp->qs_pending);
         seq_printf(m, ",%d,%llx,%d,%lu",
                    atomic_read(&rdp->dynticks->dynticks),
                    rdp->dynticks->dynticks_nesting,
@@ -186,7 +184,7 @@ static int show_rcudata_csv(struct seq_file *m, void *unused)
         int cpu;
         struct rcu_state *rsp;
 
-        seq_puts(m, "\"CPU\",\"Online?\",\"c\",\"g\",\"pq\",\"pgp\",\"pq\",");
+        seq_puts(m, "\"CPU\",\"Online?\",\"c\",\"g\",\"pq\",\"pq\",");
         seq_puts(m, "\"dt\",\"dt nesting\",\"dt NMI nesting\",\"df\",");
         seq_puts(m, "\"of\",\"qll\",\"ql\",\"qs\"");
 #ifdef CONFIG_RCU_BOOST