3 files changed, 28 insertions, 11 deletions

diff --git a/init/Kconfig b/init/Kconfig
index c26b8a1d2b57..89093e178c27 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -451,6 +451,12 @@ config RCU_USER_QS
           excluded from the global RCU state machine and thus doesn't
           to keep the timer tick on for RCU.
 
+          Unless you want to hack and help the development of the full
+          tickless feature, you shouldn't enable this option. It adds
+          unnecessary overhead.
+
+          If unsure say N
+
 config RCU_USER_QS_FORCE
         bool "Force userspace extended QS by default"
         depends on RCU_USER_QS
@@ -459,6 +465,12 @@ config RCU_USER_QS_FORCE
           test this feature that treats userspace as an extended quiescent
           state until we have a real user like a full adaptive nohz option.
 
+          Unless you want to hack and help the development of the full
+          tickless feature, you shouldn't enable this option. It adds
+          unnecessary overhead.
+
+          If unsure say N
+
 config RCU_FANOUT
         int "Tree-based hierarchical RCU fanout value"
         range 2 64 if 64BIT
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 4fb2376ddf06..74df86bd9204 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -74,6 +74,7 @@ static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
         .orphan_nxttail = &sname##_state.orphan_nxtlist, \
         .orphan_donetail = &sname##_state.orphan_donelist, \
         .barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
+        .onoff_mutex = __MUTEX_INITIALIZER(sname##_state.onoff_mutex), \
         .name = #sname, \
 }
 
@@ -1197,7 +1198,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
         raw_spin_unlock_irq(&rnp->lock);
 
         /* Exclude any concurrent CPU-hotplug operations. */
-        get_online_cpus();
+        mutex_lock(&rsp->onoff_mutex);
 
         /*
          * Set the quiescent-state-needed bits in all the rcu_node
@@ -1234,7 +1235,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
                 cond_resched();
         }
 
-        put_online_cpus();
+        mutex_unlock(&rsp->onoff_mutex);
         return 1;
 }
 
@@ -1700,6 +1701,7 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
         /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
 
         /* Exclude any attempts to start a new grace period. */
+        mutex_lock(&rsp->onoff_mutex);
         raw_spin_lock_irqsave(&rsp->onofflock, flags);
 
         /* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
@@ -1744,6 +1746,7 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
         init_callback_list(rdp);
         /* Disallow further callbacks on this CPU. */
         rdp->nxttail[RCU_NEXT_TAIL] = NULL;
+        mutex_unlock(&rsp->onoff_mutex);
 }
 
 #else /* #ifdef CONFIG_HOTPLUG_CPU */
@@ -2648,6 +2651,9 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
         struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
         struct rcu_node *rnp = rcu_get_root(rsp);
 
+        /* Exclude new grace periods. */
+        mutex_lock(&rsp->onoff_mutex);
+
         /* Set up local state, ensuring consistent view of global state. */
         raw_spin_lock_irqsave(&rnp->lock, flags);
         rdp->beenonline = 1;     /* We have now been online. */
@@ -2662,14 +2668,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
         rcu_prepare_for_idle_init(cpu);
         raw_spin_unlock(&rnp->lock);            /* irqs remain disabled. */
 
-        /*
-         * A new grace period might start here.  If so, we won't be part
-         * of it, but that is OK, as we are currently in a quiescent state.
-         */
-
-        /* Exclude any attempts to start a new GP on large systems. */
-        raw_spin_lock(&rsp->onofflock);         /* irqs already disabled. */
-
         /* Add CPU to rcu_node bitmasks. */
         rnp = rdp->mynode;
         mask = rdp->grpmask;
@@ -2693,8 +2691,9 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
                 raw_spin_unlock(&rnp->lock); /* irqs already disabled. */
                 rnp = rnp->parent;
         } while (rnp != NULL && !(rnp->qsmaskinit & mask));
+        local_irq_restore(flags);
 
-        raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+        mutex_unlock(&rsp->onoff_mutex);
 }
 
 static void __cpuinit rcu_prepare_cpu(int cpu)
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 5faf05d68326..a240f032848e 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -394,11 +394,17 @@ struct rcu_state {
         struct rcu_head **orphan_donetail;      /* Tail of above. */
         long qlen_lazy;                         /* Number of lazy callbacks. */
         long qlen;                              /* Total number of callbacks. */
+        /* End of fields guarded by onofflock. */
+
+        struct mutex onoff_mutex;               /* Coordinate hotplug & GPs. */
+
         struct mutex barrier_mutex;             /* Guards barrier fields. */
         atomic_t barrier_cpu_count;             /* # CPUs waiting on. */
         struct completion barrier_completion;   /* Wake at barrier end. */
         unsigned long n_barrier_done;           /* ++ at start and end of */
                                                 /*  _rcu_barrier(). */
+        /* End of fields guarded by barrier_mutex. */
+
         unsigned long jiffies_force_qs;         /* Time at which to invoke */
                                                 /*  force_quiescent_state(). */
         unsigned long n_force_qs;               /* Number of calls to */