cpu: Avoid puts_pending overflow

A long string of get_online_cpus() with each followed by a
put_online_cpu() that fails to acquire cpu_hotplug.lock can result in
overflow of the cpu_hotplug.puts_pending counter.  Although this is
perhaps improbably, a system with absolutely no CPU-hotplug operations
will have an arbitrarily long time in which this overflow could occur.
This commit therefore adds overflow checks to get_online_cpus() and
try_get_online_cpus().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

1 files changed, 13 insertions, 6 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 90a3d017b90c..5d220234b3ca 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -86,6 +86,16 @@ 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)
 {
         might_sleep();
@@ -93,6 +103,7 @@ void get_online_cpus(void)
                 return;
         cpuhp_lock_acquire_read();
         mutex_lock(&cpu_hotplug.lock);
+        apply_puts_pending(65536);
         cpu_hotplug.refcount++;
         mutex_unlock(&cpu_hotplug.lock);
 }
@@ -105,6 +116,7 @@ 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++;
         mutex_unlock(&cpu_hotplug.lock);
         return true;
@@ -161,12 +173,7 @@ 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;
-                }
+                apply_puts_pending(1);
                 if (likely(!cpu_hotplug.refcount))
                         break;
                 __set_current_state(TASK_UNINTERRUPTIBLE);