1 files changed, 40 insertions, 0 deletions

diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c
new file mode 100644
index 000000000000..6658d81e1836
--- /dev/null
+++ b/lib/dec_and_lock.c
@@ -0,0 +1,40 @@
+#include <linux/module.h>
+#include <linux/spinlock.h>
+#include <asm/atomic.h>
+
+/*
+ * This is an architecture-neutral, but slow,
+ * implementation of the notion of "decrement
+ * a reference count, and return locked if it
+ * decremented to zero".
+ *
+ * NOTE NOTE NOTE! This is _not_ equivalent to
+ *
+ *      if (atomic_dec_and_test(&atomic)) {
+ *              spin_lock(&lock);
+ *              return 1;
+ *      }
+ *      return 0;
+ *
+ * because the spin-lock and the decrement must be
+ * "atomic".
+ *
+ * This slow version gets the spinlock unconditionally,
+ * and releases it if it isn't needed. Architectures
+ * are encouraged to come up with better approaches,
+ * this is trivially done efficiently using a load-locked
+ * store-conditional approach, for example.
+ */
+
+#ifndef ATOMIC_DEC_AND_LOCK
+int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
+{
+        spin_lock(lock);
+        if (atomic_dec_and_test(atomic))
+                return 1;
+        spin_unlock(lock);
+        return 0;
+}
+
+EXPORT_SYMBOL(_atomic_dec_and_lock);
+#endif