filelock: new helper: vfs_inode_has_locks

Ceph has a need to know whether a particular inode has any locks set on
it. It's currently tracking that by a num_locks field in its
filp->private_data, but that's problematic as it tries to decrement this
field when releasing locks and that can race with the file being torn
down.

Add a new vfs_inode_has_locks helper that just returns whether any locks
are currently held on the inode.
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>

fs/locks.c

@@ -2672,6 +2672,29 @@ int vfs_cancel_lock(struct file *filp, struct file_lock *fl)
 }
 EXPORT_SYMBOL_GPL(vfs_cancel_lock);
 
+/**
+ * vfs_inode_has_locks - are any file locks held on @inode?
+ * @inode: inode to check for locks
+ *
+ * Return true if there are any FL_POSIX or FL_FLOCK locks currently
+ * set on @inode.
+ */
+bool vfs_inode_has_locks(struct inode *inode)
+{
+	struct file_lock_context *ctx;
+	bool ret;
+
+	ctx = smp_load_acquire(&inode->i_flctx);
+	if (!ctx)
+		return false;
+
+	spin_lock(&ctx->flc_lock);
+	ret = !list_empty(&ctx->flc_posix) || !list_empty(&ctx->flc_flock);
+	spin_unlock(&ctx->flc_lock);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(vfs_inode_has_locks);
+
 #ifdef CONFIG_PROC_FS
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>

include/linux/fs.h

@@ -1170,6 +1170,7 @@ extern int locks_delete_block(struct file_lock *);
 extern int vfs_test_lock(struct file *, struct file_lock *);
 extern int vfs_lock_file(struct file *, unsigned int, struct file_lock *, struct file_lock *);
 extern int vfs_cancel_lock(struct file *filp, struct file_lock *fl);
+bool vfs_inode_has_locks(struct inode *inode);
 extern int locks_lock_inode_wait(struct inode *inode, struct file_lock *fl);
 extern int __break_lease(struct inode *inode, unsigned int flags, unsigned int type);
 extern void lease_get_mtime(struct inode *, struct timespec64 *time);
@@ -1284,6 +1285,11 @@ static inline int vfs_cancel_lock(struct file *filp, struct file_lock *fl)
 	return 0;
 }
 
+static inline bool vfs_inode_has_locks(struct inode *inode)
+{
+	return false;
+}
+
 static inline int locks_lock_inode_wait(struct inode *inode, struct file_lock *fl)
 {
 	return -ENOLCK;