rcu: Push lock release to rcu_start_gp()'s callers

If CPUs are to give prior notice of needed grace periods, it will be
necessary to invoke rcu_start_gp() without dropping the root rcu_node
structure's ->lock.  This commit takes a second step in this direction
by moving the release of this lock to rcu_start_gp()'s callers.

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

2 files changed, 12 insertions, 17 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 2c6a9314c7f7..0d532950baa3 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1487,16 +1487,14 @@ static int __noreturn rcu_gp_kthread(void *arg)
 /*
  * Start a new RCU grace period if warranted, re-initializing the hierarchy
  * in preparation for detecting the next grace period.  The caller must hold
- * the root node's ->lock, which is released before return.  Hard irqs must
- * be disabled.
+ * the root node's ->lock and hard irqs must be disabled.
  *
  * Note that it is legal for a dying CPU (which is marked as offline) to
  * invoke this function.  This can happen when the dying CPU reports its
  * quiescent state.
  */
 static void
-rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
-        __releases(rcu_get_root(rsp)->lock)
+rcu_start_gp(struct rcu_state *rsp)
 {
         struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
         struct rcu_node *rnp = rcu_get_root(rsp);
@@ -1510,15 +1508,13 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
          */
         rcu_advance_cbs(rsp, rnp, rdp);
 
-        if (!rsp->gp_kthread ||
-            !cpu_needs_another_gp(rsp, rdp)) {
+        if (!rsp->gp_kthread || !cpu_needs_another_gp(rsp, rdp)) {
                 /*
                  * Either we have not yet spawned the grace-period
                  * task, this CPU does not need another grace period,
                  * or a grace period is already in progress.
                  * Either way, don't start a new grace period.
                  */
-                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                 return;
         }
         rsp->gp_flags = RCU_GP_FLAG_INIT;
@@ -1528,15 +1524,14 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 
         /* Wake up rcu_gp_kthread() to start the grace period. */
         wake_up(&rsp->gp_wq);
-        raw_spin_unlock_irqrestore(&rnp->lock, flags);
 }
 
 /*
  * Report a full set of quiescent states to the specified rcu_state
  * data structure.  This involves cleaning up after the prior grace
  * period and letting rcu_start_gp() start up the next grace period
- * if one is needed.  Note that the caller must hold rnp->lock, as
- * required by rcu_start_gp(), which will release it.
+ * if one is needed.  Note that the caller must hold rnp->lock, which
+ * is released before return.
  */
 static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
         __releases(rcu_get_root(rsp)->lock)
@@ -2134,7 +2129,8 @@ __rcu_process_callbacks(struct rcu_state *rsp)
         local_irq_save(flags);
         if (cpu_needs_another_gp(rsp, rdp)) {
                 raw_spin_lock(&rcu_get_root(rsp)->lock); /* irqs disabled. */
-                rcu_start_gp(rsp, flags);  /* releases above lock */
+                rcu_start_gp(rsp);
+                raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
         } else {
                 local_irq_restore(flags);
         }
@@ -2214,11 +2210,11 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
 
                 /* Start a new grace period if one not already started. */
                 if (!rcu_gp_in_progress(rsp)) {
-                        unsigned long nestflag;
                         struct rcu_node *rnp_root = rcu_get_root(rsp);
 
-                        raw_spin_lock_irqsave(&rnp_root->lock, nestflag);
-                        rcu_start_gp(rsp, nestflag);  /* rlses rnp_root->lock */
+                        raw_spin_lock(&rnp_root->lock);
+                        rcu_start_gp(rsp);
+                        raw_spin_unlock(&rnp_root->lock);
                 } else {
                         /* Give the grace period a kick. */
                         rdp->blimit = LONG_MAX;
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index df50502eca2c..073ded26e259 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2174,7 +2174,6 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp)
         unsigned long c;
         bool d;
         unsigned long flags;
-        unsigned long flags1;
         struct rcu_node *rnp = rdp->mynode;
         struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
 
@@ -2236,8 +2235,8 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp)
                                                       c, rnp->level,
                                                       rnp->grplo, rnp->grphi,
                                                       "Startedroot");
-                        local_save_flags(flags1);
-                        rcu_start_gp(rdp->rsp, flags1); /* Rlses ->lock. */
+                        rcu_start_gp(rdp->rsp);
+                        raw_spin_unlock(&rnp->lock);
                 }
 
                 /* Clean up locking and irq state. */