Merge branch 'urgent-for-mingo' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu into core/urgent

Pull two RCU fixes from Paul E. McKenney:

" - Complete the work of commit dd56af42bd82 (rcu: Eliminate deadlock
    between CPU hotplug and expedited grace periods), which was
    intended to allow synchronize_sched_expedited() to be safely
    used when holding locks acquired by CPU-hotplug notifiers.
    This commit makes the put_online_cpus() avoid the deadlock
    instead of just handling the get_online_cpus().

  - Complete the work of commit 35ce7f29a44a (rcu: Create rcuo
    kthreads only for onlined CPUs), which was intended to allow
    RCU to avoid allocating unneeded kthreads on systems where the
    firmware says that there are more CPUs than are really present.
    This commit makes rcu_barrier() aware of the mismatch, so that
    it doesn't hang waiting for non-existent CPUs. "

Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 13 insertions, 1 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);