fs: create file_write_started() helper

Convenience wrapper for sb_write_started(file_inode(inode)->i_sb)), which
has a single occurrence in the code right now.

Document the false negatives of those helpers, which makes them unusable
to assert that sb_start_write() is not held.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231122122715.2561213-16-amir73il@gmail.comReviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/read_write.c

@@ -1423,7 +1423,7 @@ ssize_t generic_copy_file_range(struct file *file_in, loff_t pos_in,
 				struct file *file_out, loff_t pos_out,
 				size_t len, unsigned int flags)
 {
-	lockdep_assert(sb_write_started(file_inode(file_out)->i_sb));
+	lockdep_assert(file_write_started(file_out));
 
 	return do_splice_direct(file_in, &pos_in, file_out, &pos_out,
 				len > MAX_RW_COUNT ? MAX_RW_COUNT : len, 0);

include/linux/fs.h

@@ -1659,11 +1659,32 @@ static inline int __sb_write_started(const struct super_block *sb, int level)
 	return lockdep_is_held_type(sb->s_writers.rw_sem + level - 1, 1);
 }
 
+/**
+ * sb_write_started - check if SB_FREEZE_WRITE is held
+ * @sb: the super we write to
+ *
+ * May be false positive with !CONFIG_LOCKDEP/LOCK_STATE_UNKNOWN.
+ */
 static inline bool sb_write_started(const struct super_block *sb)
 {
 	return __sb_write_started(sb, SB_FREEZE_WRITE);
 }
 
+/**
+ * file_write_started - check if SB_FREEZE_WRITE is held
+ * @file: the file we write to
+ *
+ * May be false positive with !CONFIG_LOCKDEP/LOCK_STATE_UNKNOWN.
+ * May be false positive with !S_ISREG, because file_start_write() has
+ * no effect on !S_ISREG.
+ */
+static inline bool file_write_started(const struct file *file)
+{
+	if (!S_ISREG(file_inode(file)->i_mode))
+		return true;
+	return sb_write_started(file_inode(file)->i_sb);
+}
+
 /**
  * sb_end_write - drop write access to a superblock
  * @sb: the super we wrote to