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>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5d22023..1972b16 100644 (file)
--- 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)