1 files changed, 35 insertions, 0 deletions
diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c
new file mode 100644
index 0000000..a65c314
--- /dev/null
+++ b/lib/dec_and_lock.c
@@ -0,0 +1,35 @@
+#include <linux/module.h>
+#include <linux/spinlock.h>
+#include <asm/atomic.h>
+
+/*
+ * This is an 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".
+ */
+int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
+{
+#ifdef CONFIG_SMP
+	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
+	if (atomic_add_unless(atomic, -1, 1))
+		return 0;
+#endif
+	/* Otherwise do it the slow way */
+	spin_lock(lock);
+	if (atomic_dec_and_test(atomic))
+		return 1;
+	spin_unlock(lock);
+	return 0;
+}
+
+EXPORT_SYMBOL(_atomic_dec_and_lock);