[RFC PATCH] filelock: new helper: vfs_file_has_locks

[Jeff Layton <jlayton@kernel.org>]

Ceph has a need to know whether a particular file 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_file_has_locks helper that will scan the flock and posix
lists, and return true if any of the locks have a fl_file that matches
the given one. Ceph can then call this instead of doing its own
tracking.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/locks.c         | 36 ++++++++++++++++++++++++++++++++++++
 include/linux/fs.h |  1 +
 2 files changed, 37 insertions(+)

Xiubo,

Here's what I was thinking instead of trying to track this within ceph.
Most inodes never have locks set, so in most cases this will be a NULL
pointer check.

diff --git a/fs/locks.c b/fs/locks.c
index 5876c8ff0edc..c7f903b63a53 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -2672,6 +2672,42 @@ int vfs_cancel_lock(struct file *filp, struct file_lock *fl)
 }
 EXPORT_SYMBOL_GPL(vfs_cancel_lock);
 
+/**
+ * vfs_file_has_locks - are any locks held that were set on @filp?
+ * @filp: open file to check for locks
+ *
+ * Return true if are any FL_POSIX or FL_FLOCK locks currently held
+ * on @filp.
+ */
+bool vfs_file_has_locks(struct file *filp)
+{
+	struct file_lock_context *ctx;
+	struct file_lock *fl;
+	bool ret = false;
+
+	ctx = smp_load_acquire(&locks_inode(filp)->i_flctx);
+	if (!ctx)
+		return false;
+
+	spin_lock(&ctx->flc_lock);
+	list_for_each_entry(fl, &ctx->flc_posix, fl_list) {
+		if (fl->fl_file == filp) {
+			ret = true;
+			goto out;
+		}
+	}
+	list_for_each_entry(fl, &ctx->flc_flock, fl_list) {
+		if (fl->fl_file == filp) {
+			ret = true;
+			break;
+		}
+	}
+out:
+	spin_unlock(&ctx->flc_lock);
+	return ret;
+}
+EXPORT_SYMBOL(vfs_file_has_locks);
+
 #ifdef CONFIG_PROC_FS
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e654435f1651..e4d0f1fa7f9f 100644
--- a/include/linux/fs.h
+++ b/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_file_has_locks(struct file *file);
 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);
-- 
2.38.1