rcu: Move propagation of ->completed from rcu_start_gp() to rcu_report_qs_rsp()

It is possible for the CPU that noted the end of the prior grace period
to not need a new one, and therefore to decide to propagate ->completed
throughout the rcu_node tree without starting another grace period.
However, in so doing, it releases the root rcu_node structure's lock,
which can allow some other CPU to start another grace period.  The first
CPU will be propagating ->completed in parallel with the second CPU
initializing the rcu_node tree for the new grace period.  In theory
this is harmless, but in practice we need to keep things simple.

This commit therefore moves the propagation of ->completed to
rcu_report_qs_rsp(), and refrains from marking the old grace period
as having been completed until it has finished doing this.  This
prevents anyone from starting a new grace period concurrently with
marking the old grace period as having been completed.

Of course, the optimization where a CPU needing a new grace period
doesn't bother marking the old one completed is still in effect:
In that case, the marking happens implicitly as part of initializing
the new grace period.

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

1 files changed, 51 insertions, 20 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index e75df0c93abd..e234eb92a177 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -842,28 +842,24 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
         struct rcu_node *rnp = rcu_get_root(rsp);
 
         if (!rcu_scheduler_fully_active ||
-            !cpu_needs_another_gp(rsp, rdp) ||
-            rsp->fqs_active) {
-                if (rcu_scheduler_fully_active &&
-                    cpu_needs_another_gp(rsp, rdp))
-                        rsp->fqs_need_gp = 1;
-                if (rnp->completed == rsp->completed) {
-                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
-                        return;
-                }
-                raw_spin_unlock(&rnp->lock);     /* irqs remain disabled. */
+            !cpu_needs_another_gp(rsp, rdp)) {
+                /*
+                 * Either the scheduler hasn't yet spawned the first
+                 * non-idle task or this CPU does not need another
+                 * grace period.  Either way, don't start a new grace
+                 * period.
+                 */
+                raw_spin_unlock_irqrestore(&rnp->lock, flags);
+                return;
+        }
 
+        if (rsp->fqs_active) {
                 /*
-                 * Propagate new ->completed value to rcu_node structures
-                 * so that other CPUs don't have to wait until the start
-                 * of the next grace period to process their callbacks.
+                 * This CPU needs a grace period, but force_quiescent_state()
+                 * is running.  Tell it to start one on this CPU's behalf.
                  */
-                rcu_for_each_node_breadth_first(rsp, rnp) {
-                        raw_spin_lock(&rnp->lock); /* irqs already disabled. */
-                        rnp->completed = rsp->completed;
-                        raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
-                }
-                local_irq_restore(flags);
+                rsp->fqs_need_gp = 1;
+                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                 return;
         }
 
@@ -947,6 +943,8 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
         __releases(rcu_get_root(rsp)->lock)
 {
         unsigned long gp_duration;
+        struct rcu_node *rnp = rcu_get_root(rsp);
+        struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
 
         WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
 
@@ -958,7 +956,40 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
         gp_duration = jiffies - rsp->gp_start;
         if (gp_duration > rsp->gp_max)
                 rsp->gp_max = gp_duration;
-        rsp->completed = rsp->gpnum;
+
+        /*
+         * We know the grace period is complete, but to everyone else
+         * it appears to still be ongoing.  But it is also the case
+         * that to everyone else it looks like there is nothing that
+         * they can do to advance the grace period.  It is therefore
+         * safe for us to drop the lock in order to mark the grace
+         * period as completed in all of the rcu_node structures.
+         *
+         * But if this CPU needs another grace period, it will take
+         * care of this while initializing the next grace period.
+         * We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
+         * because the callbacks have not yet been advanced: Those
+         * callbacks are waiting on the grace period that just now
+         * completed.
+         */
+        if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
+                raw_spin_unlock(&rnp->lock);     /* irqs remain disabled. */
+
+                /*
+                 * Propagate new ->completed value to rcu_node structures
+                 * so that other CPUs don't have to wait until the start
+                 * of the next grace period to process their callbacks.
+                 */
+                rcu_for_each_node_breadth_first(rsp, rnp) {
+                        raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+                        rnp->completed = rsp->gpnum;
+                        raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+                }
+                rnp = rcu_get_root(rsp);
+                raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+        }
+
+        rsp->completed = rsp->gpnum;  /* Declare the grace period complete. */
         trace_rcu_grace_period(rsp->name, rsp->completed, "end");
         rsp->signaled = RCU_GP_IDLE;
         rcu_start_gp(rsp, flags);  /* releases root node's rnp->lock. */