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-rw-r--r--lib/dec_and_lock.c51
1 files changed, 51 insertions, 0 deletions
diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c
new file mode 100644
index 000000000..9555b68bb
--- /dev/null
+++ b/lib/dec_and_lock.c
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/export.h>
+#include <linux/spinlock.h>
+#include <linux/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)
+{
+ /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
+ if (atomic_add_unless(atomic, -1, 1))
+ return 0;
+
+ /* 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);
+
+int _atomic_dec_and_lock_irqsave(atomic_t *atomic, spinlock_t *lock,
+ unsigned long *flags)
+{
+ /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
+ if (atomic_add_unless(atomic, -1, 1))
+ return 0;
+
+ /* Otherwise do it the slow way */
+ spin_lock_irqsave(lock, *flags);
+ if (atomic_dec_and_test(atomic))
+ return 1;
+ spin_unlock_irqrestore(lock, *flags);
+ return 0;
+}
+EXPORT_SYMBOL(_atomic_dec_and_lock_irqsave);