[PATCH 3/6] btrfs: introduce try-lock semantics for exclusive op start

[David Sterba <dsterba@suse.com>]

Add try-lock for exclusive operation start to allow callers to do more
checks. The same operation must already be running. The try-lock and
unlock must pair and are a substitute for btrfs_exclop_start, thus it
must also pair with btrfs_exclop_finish to release the exclop context.

Signed-off-by: David Sterba <dsterba@suse.com>
---
 fs/btrfs/ctree.h |  3 +++
 fs/btrfs/ioctl.c | 26 ++++++++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 3dfc32a3ebab..0dffc06b5ad4 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -3231,6 +3231,9 @@ void btrfs_update_ioctl_balance_args(struct btrfs_fs_info *fs_info,
 			       struct btrfs_ioctl_balance_args *bargs);
 bool btrfs_exclop_start(struct btrfs_fs_info *fs_info,
 			enum btrfs_exclusive_operation type);
+bool btrfs_exclop_start_try_lock(struct btrfs_fs_info *fs_info,
+				 enum btrfs_exclusive_operation type);
+void btrfs_exclop_start_unlock(struct btrfs_fs_info *fs_info);
 void btrfs_exclop_finish(struct btrfs_fs_info *fs_info);
 
 /* file.c */
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index c4e710ea08ba..cacd6ee17d8e 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -371,6 +371,32 @@ bool btrfs_exclop_start(struct btrfs_fs_info *fs_info,
 	return ret;
 }
 
+/*
+ * Conditionally allow to enter the exclusive operation in case it's compatible
+ * with the running one.  This must be paired with btrfs_exclop_start_unlock and
+ * btrfs_exclop_finish.
+ *
+ * Compatibility:
+ * - the same type is already running
+ * - not BTRFS_EXCLOP_NONE - this is intentionally incompatible and the caller
+ *   must check the condition first that would allow none -> @type
+ */
+bool btrfs_exclop_start_try_lock(struct btrfs_fs_info *fs_info,
+				 enum btrfs_exclusive_operation type)
+{
+	spin_lock(&fs_info->super_lock);
+	if (fs_info->exclusive_operation == type)
+		return true;
+
+	spin_unlock(&fs_info->super_lock);
+	return false;
+}
+
+void btrfs_exclop_start_unlock(struct btrfs_fs_info *fs_info)
+{
+	spin_unlock(&fs_info->super_lock);
+}
+
 void btrfs_exclop_finish(struct btrfs_fs_info *fs_info)
 {
 	spin_lock(&fs_info->super_lock);
-- 
2.29.2