Merge branch 'core-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull core fixes from Ingo Molnar:
 "The tree contains two RCU fixes and a compiler quirk comment fix"

* 'core-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  rcu: Make rcu_barrier() understand about missing rcuo kthreads
  compiler/gcc4+: Remove inaccurate comment about 'asm goto' miscompiles
  rcu: More on deadlock between CPU hotplug and expedited grace periods

4 files changed, 57 insertions, 6 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 356450f09c1f..90a3d017b90c 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -64,6 +64,8 @@ static struct {
          * an ongoing cpu hotplug operation.
          */
         int refcount;
+        /* And allows lockless put_online_cpus(). */
+        atomic_t puts_pending;
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
         struct lockdep_map dep_map;
@@ -113,7 +115,11 @@ void put_online_cpus(void)
 {
         if (cpu_hotplug.active_writer == current)
                 return;
-        mutex_lock(&cpu_hotplug.lock);
+        if (!mutex_trylock(&cpu_hotplug.lock)) {
+                atomic_inc(&cpu_hotplug.puts_pending);
+                cpuhp_lock_release();
+                return;
+        }
 
         if (WARN_ON(!cpu_hotplug.refcount))
                 cpu_hotplug.refcount++; /* try to fix things up */
@@ -155,6 +161,12 @@ void cpu_hotplug_begin(void)
         cpuhp_lock_acquire();
         for (;;) {
                 mutex_lock(&cpu_hotplug.lock);
+                if (atomic_read(&cpu_hotplug.puts_pending)) {
+                        int delta;
+
+                        delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
+                        cpu_hotplug.refcount -= delta;
+                }
                 if (likely(!cpu_hotplug.refcount))
                         break;
                 __set_current_state(TASK_UNINTERRUPTIBLE);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 133e47223095..9815447d22e0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3299,11 +3299,16 @@ static void _rcu_barrier(struct rcu_state *rsp)
                         continue;
                 rdp = per_cpu_ptr(rsp->rda, cpu);
                 if (rcu_is_nocb_cpu(cpu)) {
-                        _rcu_barrier_trace(rsp, "OnlineNoCB", cpu,
-                                           rsp->n_barrier_done);
-                        atomic_inc(&rsp->barrier_cpu_count);
-                        __call_rcu(&rdp->barrier_head, rcu_barrier_callback,
-                                   rsp, cpu, 0);
+                        if (!rcu_nocb_cpu_needs_barrier(rsp, cpu)) {
+                                _rcu_barrier_trace(rsp, "OfflineNoCB", cpu,
+                                                   rsp->n_barrier_done);
+                        } else {
+                                _rcu_barrier_trace(rsp, "OnlineNoCB", cpu,
+                                                   rsp->n_barrier_done);
+                                atomic_inc(&rsp->barrier_cpu_count);
+                                __call_rcu(&rdp->barrier_head,
+                                           rcu_barrier_callback, rsp, cpu, 0);
+                        }
                 } else if (ACCESS_ONCE(rdp->qlen)) {
                         _rcu_barrier_trace(rsp, "OnlineQ", cpu,
                                            rsp->n_barrier_done);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index d03764652d91..bbdc45d8d74f 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -587,6 +587,7 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
 static void print_cpu_stall_info_end(void);
 static void zero_cpu_stall_ticks(struct rcu_data *rdp);
 static void increment_cpu_stall_ticks(void);
+static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu);
 static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq);
 static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp);
 static void rcu_init_one_nocb(struct rcu_node *rnp);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 387dd4599344..c1d7f27bd38f 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2050,6 +2050,33 @@ static void wake_nocb_leader(struct rcu_data *rdp, bool force)
 }
 
 /*
+ * Does the specified CPU need an RCU callback for the specified flavor
+ * of rcu_barrier()?
+ */
+static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu)
+{
+        struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
+        struct rcu_head *rhp;
+
+        /* No-CBs CPUs might have callbacks on any of three lists. */
+        rhp = ACCESS_ONCE(rdp->nocb_head);
+        if (!rhp)
+                rhp = ACCESS_ONCE(rdp->nocb_gp_head);
+        if (!rhp)
+                rhp = ACCESS_ONCE(rdp->nocb_follower_head);
+
+        /* Having no rcuo kthread but CBs after scheduler starts is bad! */
+        if (!ACCESS_ONCE(rdp->nocb_kthread) && rhp) {
+                /* RCU callback enqueued before CPU first came online??? */
+                pr_err("RCU: Never-onlined no-CBs CPU %d has CB %p\n",
+                       cpu, rhp->func);
+                WARN_ON_ONCE(1);
+        }
+
+        return !!rhp;
+}
+
+/*
  * Enqueue the specified string of rcu_head structures onto the specified
  * CPU's no-CBs lists.  The CPU is specified by rdp, the head of the
  * string by rhp, and the tail of the string by rhtp.  The non-lazy/lazy
@@ -2642,6 +2669,12 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
 
 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
 
+static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu)
+{
+        WARN_ON_ONCE(1); /* Should be dead code. */
+        return false;
+}
+
 static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
 {
 }