hotplugcpu: Avoid deadlocks by waking active_writer

Commit b2c4623dcd07 ("rcu: More on deadlock between CPU hotplug and expedited
grace periods") introduced another problem that can easily be reproduced by
starting/stopping cpus in a loop.

E.g.:
  for i in `seq 5000`; do
      echo 1 > /sys/devices/system/cpu/cpu1/online
      echo 0 > /sys/devices/system/cpu/cpu1/online
  done

Will result in:
  INFO: task /cpu_start_stop:1 blocked for more than 120 seconds.
  Call Trace:
  ([<00000000006a028e>] __schedule+0x406/0x91c)
   [<0000000000130f60>] cpu_hotplug_begin+0xd0/0xd4
   [<0000000000130ff6>] _cpu_up+0x3e/0x1c4
   [<0000000000131232>] cpu_up+0xb6/0xd4
   [<00000000004a5720>] device_online+0x80/0xc0
   [<00000000004a57f0>] online_store+0x90/0xb0
  ...

And a deadlock.

Problem is that if the last ref in put_online_cpus() can't get the
cpu_hotplug.lock the puts_pending count is incremented, but a sleeping
active_writer might never be woken up, therefore never exiting the loop in
cpu_hotplug_begin().

This fix removes puts_pending and turns refcount into an atomic variable. We
also introduce a wait queue for the active_writer, to avoid possible races and
use-after-free. There is no need to take the lock in put_online_cpus() anymore.

Can't reproduce it with this fix.

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

1 files changed, 23 insertions, 33 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5d220234b3ca..1972b161c61e 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -58,22 +58,23 @@ static int cpu_hotplug_disabled;
 
 static struct {
         struct task_struct *active_writer;
-        struct mutex lock; /* Synchronizes accesses to refcount, */
+        /* wait queue to wake up the active_writer */
+        wait_queue_head_t wq;
+        /* verifies that no writer will get active while readers are active */
+        struct mutex lock;
         /*
          * Also blocks the new readers during
          * an ongoing cpu hotplug operation.
          */
-        int refcount;
-        /* And allows lockless put_online_cpus(). */
-        atomic_t puts_pending;
+        atomic_t refcount;
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
         struct lockdep_map dep_map;
 #endif
 } cpu_hotplug = {
         .active_writer = NULL,
+        .wq = __WAIT_QUEUE_HEAD_INITIALIZER(cpu_hotplug.wq),
         .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
-        .refcount = 0,
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
         .dep_map = {.name = "cpu_hotplug.lock" },
 #endif
@@ -86,15 +87,6 @@ static struct {
 #define cpuhp_lock_acquire()      lock_map_acquire(&cpu_hotplug.dep_map)
 #define cpuhp_lock_release()      lock_map_release(&cpu_hotplug.dep_map)
 
-static void apply_puts_pending(int max)
-{
-        int delta;
-
-        if (atomic_read(&cpu_hotplug.puts_pending) >= max) {
-                delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
-                cpu_hotplug.refcount -= delta;
-        }
-}
 
 void get_online_cpus(void)
 {
@@ -103,8 +95,7 @@ void get_online_cpus(void)
                 return;
         cpuhp_lock_acquire_read();
         mutex_lock(&cpu_hotplug.lock);
-        apply_puts_pending(65536);
-        cpu_hotplug.refcount++;
+        atomic_inc(&cpu_hotplug.refcount);
         mutex_unlock(&cpu_hotplug.lock);
 }
 EXPORT_SYMBOL_GPL(get_online_cpus);
@@ -116,8 +107,7 @@ bool try_get_online_cpus(void)
         if (!mutex_trylock(&cpu_hotplug.lock))
                 return false;
         cpuhp_lock_acquire_tryread();
-        apply_puts_pending(65536);
-        cpu_hotplug.refcount++;
+        atomic_inc(&cpu_hotplug.refcount);
         mutex_unlock(&cpu_hotplug.lock);
         return true;
 }
@@ -125,20 +115,18 @@ EXPORT_SYMBOL_GPL(try_get_online_cpus);
 
 void put_online_cpus(void)
 {
+        int refcount;
+
         if (cpu_hotplug.active_writer == current)
                 return;
-        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 */
+        refcount = atomic_dec_return(&cpu_hotplug.refcount);
+        if (WARN_ON(refcount < 0)) /* try to fix things up */
+                atomic_inc(&cpu_hotplug.refcount);
+
+        if (refcount <= 0 && waitqueue_active(&cpu_hotplug.wq))
+                wake_up(&cpu_hotplug.wq);
 
-        if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
-                wake_up_process(cpu_hotplug.active_writer);
-        mutex_unlock(&cpu_hotplug.lock);
         cpuhp_lock_release();
 
 }
@@ -168,18 +156,20 @@ EXPORT_SYMBOL_GPL(put_online_cpus);
  */
 void cpu_hotplug_begin(void)
 {
-        cpu_hotplug.active_writer = current;
+        DEFINE_WAIT(wait);
 
+        cpu_hotplug.active_writer = current;
         cpuhp_lock_acquire();
+
         for (;;) {
                 mutex_lock(&cpu_hotplug.lock);
-                apply_puts_pending(1);
-                if (likely(!cpu_hotplug.refcount))
-                        break;
-                __set_current_state(TASK_UNINTERRUPTIBLE);
+                prepare_to_wait(&cpu_hotplug.wq, &wait, TASK_UNINTERRUPTIBLE);
+                if (likely(!atomic_read(&cpu_hotplug.refcount)))
+                                break;
                 mutex_unlock(&cpu_hotplug.lock);
                 schedule();
         }
+        finish_wait(&cpu_hotplug.wq, &wait);
 }
 
 void cpu_hotplug_done(void)