vfs: get rid of old '->iterate' directory operation

All users now just use '->iterate_shared()', which only takes the
directory inode lock for reading.

Filesystems that never got convered to shared mode now instead use a
wrapper that drops the lock, re-takes it in write mode, calls the old
function, and then downgrades the lock back to read mode.

This way the VFS layer and other callers no longer need to care about
filesystems that never got converted to the modern era.

The filesystems that use the new wrapper are ceph, coda, exfat, jfs,
ntfs, ocfs2, overlayfs, and vboxsf.

Honestly, several of them look like they really could just iterate their
directories in shared mode and skip the wrapper entirely, but the point
of this change is to not change semantics or fix filesystems that
haven't been fixed in the last 7+ years, but to finally get rid of the
dual iterators.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

2 files changed, 12 insertions, 18 deletions

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index ed148919e11ad5..0ca479dbb1cd1f 100644
--- a/Documentation/filesystems/locking.rst
+++ b/Documentation/filesystems/locking.rst
@@ -551,9 +551,8 @@ mutex or just to use i_size_read() instead.
 Note: this does not protect the file->f_pos against concurrent modifications
 since this is something the userspace has to take care about.
 
-->iterate() is called with i_rwsem exclusive.
-
-->iterate_shared() is called with i_rwsem at least shared.
+->iterate_shared() is called with i_rwsem held for reading, and with the
+file f_pos_lock held exclusively
 
 ->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
 Most instances call fasync_helper(), which does that maintenance, so it's
diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
index d2d684ae77984f..0f5da78ef4f905 100644
--- a/Documentation/filesystems/porting.rst
+++ b/Documentation/filesystems/porting.rst
@@ -537,7 +537,7 @@ vfs_readdir() is gone; switch to iterate_dir() instead
 
 **mandatory**
 
-->readdir() is gone now; switch to ->iterate()
+->readdir() is gone now; switch to ->iterate_shared()
 
 **mandatory**
 
@@ -693,24 +693,19 @@ parallel now.
 
 ---
 
-**recommended**
+**mandatory**
 
-->iterate_shared() is added; it's a parallel variant of ->iterate().
+->iterate_shared() is added.
 Exclusion on struct file level is still provided (as well as that
 between it and lseek on the same struct file), but if your directory
 has been opened several times, you can get these called in parallel.
 Exclusion between that method and all directory-modifying ones is
 still provided, of course.
 
-Often enough ->iterate() can serve as ->iterate_shared() without any
-changes - it is a read-only operation, after all.  If you have any
-per-inode or per-dentry in-core data structures modified by ->iterate(),
-you might need something to serialize the access to them.  If you
-do dcache pre-seeding, you'll need to switch to d_alloc_parallel() for
-that; look for in-tree examples.
-
-Old method is only used if the new one is absent; eventually it will
-be removed.  Switch while you still can; the old one won't stay.
+If you have any per-inode or per-dentry in-core data structures modified
+by ->iterate_shared(), you might need something to serialize the access
+to them.  If you do dcache pre-seeding, you'll need to switch to
+d_alloc_parallel() for that; look for in-tree examples.
 
 ---
 
@@ -930,9 +925,9 @@ should be done by looking at FMODE_LSEEK in file->f_mode.
 filldir_t (readdir callbacks) calling conventions have changed.  Instead of
 returning 0 or -E... it returns bool now.  false means "no more" (as -E... used
 to) and true - "keep going" (as 0 in old calling conventions).  Rationale:
-callers never looked at specific -E... values anyway.  ->iterate() and
-->iterate_shared() instance require no changes at all, all filldir_t ones in
-the tree converted.
+callers never looked at specific -E... values anyway. -> iterate_shared()
+instances require no changes at all, all filldir_t ones in the tree
+converted.
 
 ---