btrfs: Implement DREW lock

A (D)ouble (R)eader (W)riter (E)xclustion lock is a locking primitive
that allows to have multiple readers or multiple writers but not
multiple readers and writers holding it concurrently.

The code is factored out from the existing open-coded locking scheme
used to exclude pending snapshots from nocow writers and vice-versa.
Current implementation actually favors Readers (that is snapshot
creaters) to writers (nocow writers of the filesystem).

The API provides lock/unlock/trylock for reads and writes.

Formal specification for TLA+ provided by Valentin Schneider is at
https://lore.kernel.org/linux-btrfs/2dcaf81c-f0d3-409e-cb29-733d8b3b4cc9@arm.com/

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 93 insertions, 0 deletions

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index e713900f96b6..fb647d8cf527 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -565,3 +565,96 @@ struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
 	}
 	return eb;
 }
+
+/*
+ * DREW locks
+ * ==========
+ *
+ * DREW stands for double-reader-writer-exclusion lock. It's used in situation
+ * where you want to provide A-B exclusion but not AA or BB.
+ *
+ * Currently implementation gives more priority to reader. If a reader and a
+ * writer both race to acquire their respective sides of the lock the writer
+ * would yield its lock as soon as it detects a concurrent reader. Additionally
+ * if there are pending readers no new writers would be allowed to come in and
+ * acquire the lock.
+ */
+
+int btrfs_drew_lock_init(struct btrfs_drew_lock *lock)
+{
+	int ret;
+
+	ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
+	if (ret)
+		return ret;
+
+	atomic_set(&lock->readers, 0);
+	init_waitqueue_head(&lock->pending_readers);
+	init_waitqueue_head(&lock->pending_writers);
+
+	return 0;
+}
+
+void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock)
+{
+	percpu_counter_destroy(&lock->writers);
+}
+
+/* Return true if acquisition is successful, false otherwise */
+bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock)
+{
+	if (atomic_read(&lock->readers))
+		return false;
+
+	percpu_counter_inc(&lock->writers);
+
+	/* Ensure writers count is updated before we check for pending readers */
+	smp_mb();
+	if (atomic_read(&lock->readers)) {
+		btrfs_drew_write_unlock(lock);
+		return false;
+	}
+
+	return true;
+}
+
+void btrfs_drew_write_lock(struct btrfs_drew_lock *lock)
+{
+	while (true) {
+		if (btrfs_drew_try_write_lock(lock))
+			return;
+		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
+	}
+}
+
+void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock)
+{
+	percpu_counter_dec(&lock->writers);
+	cond_wake_up(&lock->pending_readers);
+}
+
+void btrfs_drew_read_lock(struct btrfs_drew_lock *lock)
+{
+	atomic_inc(&lock->readers);
+
+	/*
+	 * Ensure the pending reader count is perceieved BEFORE this reader
+	 * goes to sleep in case of active writers. This guarantees new writers
+	 * won't be allowed and that the current reader will be woken up when
+	 * the last active writer finishes its jobs.
+	 */
+	smp_mb__after_atomic();
+
+	wait_event(lock->pending_readers,
+		   percpu_counter_sum(&lock->writers) == 0);
+}
+
+void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock)
+{
+	/*
+	 * atomic_dec_and_test implies a full barrier, so woken up writers
+	 * are guaranteed to see the decrement
+	 */
+	if (atomic_dec_and_test(&lock->readers))
+		wake_up(&lock->pending_writers);
+}