rcu: Introduce grace-period sequence numbers

This commit adds grace-period sequence numbers (->gp_seq) to the
rcu_state, rcu_node, and rcu_data structures, and updates them.
It also checks for consistency between rsp->gpnum and rsp->gp_seq.
These ->gp_seq counters will eventually replace the existing ->gpnum
and ->completed counters, allowing a single memory access to determine
whether or not a grace period is in progress and if so, which one.
This in turn will enable changes that will reduce ->lock contention on
the leaf rcu_node structures.

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

1 files changed, 15 insertions, 1 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a9a4a260ea7d..467cd8e5c6ff 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -97,6 +97,7 @@ struct rcu_state sname##_state = { \
         .gp_state = RCU_GP_IDLE, \
         .gpnum = 0UL - 300UL, \
         .completed = 0UL - 300UL, \
+        .gp_seq = (0UL - 300UL) << RCU_SEQ_CTR_SHIFT, \
         .barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
         .name = RCU_STATE_NAME(sname), \
         .abbr = sabbr, \
@@ -1849,6 +1850,8 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
                 WRITE_ONCE(rdp->gpwrap, false);
                 rcu_gpnum_ovf(rnp, rdp);
         }
+        if (rdp->gp_seq != rnp->gp_seq)
+                rdp->gp_seq = rnp->gp_seq;
         return ret;
 }
 
@@ -1910,7 +1913,10 @@ static bool rcu_gp_init(struct rcu_state *rsp)
         /* Advance to a new grace period and initialize state. */
         record_gp_stall_check_time(rsp);
         /* Record GP times before starting GP, hence smp_store_release(). */
+        WARN_ON_ONCE(rsp->gpnum << RCU_SEQ_CTR_SHIFT != rsp->gp_seq);
         smp_store_release(&rsp->gpnum, rsp->gpnum + 1);
+        smp_mb(); /* Pairs with barriers in stall-warning code. */
+        rcu_seq_start(&rsp->gp_seq);
         trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));
         raw_spin_unlock_irq_rcu_node(rnp);
 
@@ -1984,6 +1990,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
                 WRITE_ONCE(rnp->gpnum, rsp->gpnum);
                 if (WARN_ON_ONCE(rnp->completed != rsp->completed))
                         WRITE_ONCE(rnp->completed, rsp->completed);
+                WRITE_ONCE(rnp->gp_seq, rsp->gp_seq);
                 if (rnp == rdp->mynode)
                         (void)__note_gp_changes(rsp, rnp, rdp);
                 rcu_preempt_boost_start_gp(rnp);
@@ -2050,6 +2057,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 {
         unsigned long gp_duration;
         bool needgp = false;
+        unsigned long new_gp_seq;
         struct rcu_data *rdp;
         struct rcu_node *rnp = rcu_get_root(rsp);
         struct swait_queue_head *sq;
@@ -2079,12 +2087,15 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
          * all of the rcu_node structures before the beginning of the next
          * grace period is recorded in any of the rcu_node structures.
          */
+        new_gp_seq = rsp->gp_seq;
+        rcu_seq_end(&new_gp_seq);
         rcu_for_each_node_breadth_first(rsp, rnp) {
                 raw_spin_lock_irq_rcu_node(rnp);
                 if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
                         dump_blkd_tasks(rnp, 10);
                 WARN_ON_ONCE(rnp->qsmask);
                 WRITE_ONCE(rnp->completed, rsp->gpnum);
+                WRITE_ONCE(rnp->gp_seq, new_gp_seq);
                 rdp = this_cpu_ptr(rsp->rda);
                 if (rnp == rdp->mynode)
                         needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
@@ -2098,10 +2109,11 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
                 rcu_gp_slow(rsp, gp_cleanup_delay);
         }
         rnp = rcu_get_root(rsp);
-        raw_spin_lock_irq_rcu_node(rnp); /* Order GP before ->completed update. */
+        raw_spin_lock_irq_rcu_node(rnp); /* GP before rsp->gp_seq update. */
 
         /* Declare grace period done. */
         WRITE_ONCE(rsp->completed, rsp->gpnum);
+        rcu_seq_end(&rsp->gp_seq);
         trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
         rsp->gp_state = RCU_GP_IDLE;
         /* Check for GP requests since above loop. */
@@ -3612,6 +3624,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
         rdp->beenonline = true;  /* We have now been online. */
         rdp->gpnum = rnp->completed; /* Make CPU later note any new GP. */
         rdp->completed = rnp->completed;
+        rdp->gp_seq = rnp->gp_seq;
         rdp->cpu_no_qs.b.norm = true;
         rdp->rcu_qs_ctr_snap = per_cpu(rcu_dynticks.rcu_qs_ctr, cpu);
         rdp->core_needs_qs = false;
@@ -3991,6 +4004,7 @@ static void __init rcu_init_one(struct rcu_state *rsp)
                                                    &rcu_fqs_class[i], fqs[i]);
                         rnp->gpnum = rsp->gpnum;
                         rnp->completed = rsp->completed;
+                        rnp->gp_seq = rsp->gp_seq;
                         rnp->completedqs = rsp->completed;
                         rnp->qsmask = 0;
                         rnp->qsmaskinit = 0;