[PATCH v2] fs: drop assert in file_seek_cur_needs_f_lock

[Luis Henriques <luis@igalia.com>]

The assert in function file_seek_cur_needs_f_lock() can be triggered very
easily because there are many users of vfs_llseek() (such as overlayfs)
that do their custom locking around llseek instead of relying on
fdget_pos(). Just drop the overzealous assertion.

Fixes: da06e3c51794 ("fs: don't needlessly acquire f_lock")
Suggested-by: Jan Kara <jack@suse.cz>
Suggested-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Luis Henriques <luis@igalia.com>
---
Hi!

As suggested by Mateusz, I'm adding a comment (also suggested by him!) to
replace the assertion.  I'm also adding the 'Suggested-by:' tags, although
I'm not sure it's the correct tag to use -- the authorship of this patch
isn't really clear at this point :-)

 fs/file.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 3a3146664cf3..b6db031545e6 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1198,8 +1198,12 @@ bool file_seek_cur_needs_f_lock(struct file *file)
 	if (!(file->f_mode & FMODE_ATOMIC_POS) && !file->f_op->iterate_shared)
 		return false;
 
-	VFS_WARN_ON_ONCE((file_count(file) > 1) &&
-			 !mutex_is_locked(&file->f_pos_lock));
+	/*
+	 * Note that we are not guaranteed to be called after fdget_pos() on
+	 * this file obj, in which case the caller is expected to provide the
+	 * appropriate locking.
+	 */
+
 	return true;
 }