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>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 90a3d01..5d22023 100644 (file)
--- 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);