bcachefs: Factor out two_state_shared_lock

We have a unique lock used for controlling adding to the pagecache: the
lock has two states, where both states are shared - the lock may be held
multiple times for either state - but not both states at the same time.

This is exactly what we need for nocow mode locking, so this patch pulls
it out of fs.c into its own file.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 33 insertions, 0 deletions

diff --git a/fs/bcachefs/two_state_shared_lock.c b/fs/bcachefs/two_state_shared_lock.c
new file mode 100644
index 000000000000..dc508d545de0
--- /dev/null
+++ b/fs/bcachefs/two_state_shared_lock.c
@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "two_state_shared_lock.h"
+
+void bch2_two_state_unlock(two_state_lock_t *lock, int s)
+{
+	long i = s ? 1 : -1;
+
+	BUG_ON(atomic_long_read(&lock->v) == 0);
+
+	if (atomic_long_sub_return_release(i, &lock->v) == 0)
+		wake_up_all(&lock->wait);
+}
+
+bool bch2_two_state_trylock(two_state_lock_t *lock, int s)
+{
+	long i = s ? 1 : -1;
+	long v = atomic_long_read(&lock->v), old;
+
+	do {
+		old = v;
+
+		if (i > 0 ? v < 0 : v > 0)
+			return false;
+	} while ((v = atomic_long_cmpxchg_acquire(&lock->v,
+					old, old + i)) != old);
+	return true;
+}
+
+void bch2_two_state_lock(two_state_lock_t *lock, int s)
+{
+	wait_event(lock->wait, bch2_two_state_trylock(lock, s));
+}