rename(): fix the locking of subdirectories

	We should never lock two subdirectories without having taken
->s_vfs_rename_mutex; inode pointer order or not, the "order" proposed
in 28eceeda130f "fs: Lock moved directories" is not transitive, with
the usual consequences.

	The rationale for locking renamed subdirectory in all cases was
the possibility of race between rename modifying .. in a subdirectory to
reflect the new parent and another thread modifying the same subdirectory.
For a lot of filesystems that's not a problem, but for some it can lead
to trouble (e.g. the case when short directory contents is kept in the
inode, but creating a file in it might push it across the size limit
and copy its contents into separate data block(s)).

	However, we need that only in case when the parent does change -
otherwise ->rename() doesn't need to do anything with .. entry in the
first place.  Some instances are lazy and do a tautological update anyway,
but it's really not hard to avoid.

Amended locking rules for rename():
	find the parent(s) of source and target
	if source and target have the same parent
		lock the common parent
	else
		lock ->s_vfs_rename_mutex
		lock both parents, in ancestor-first order; if neither
		is an ancestor of another, lock the parent of source
		first.
	find the source and target.
	if source and target have the same parent
		if operation is an overwriting rename of a subdirectory
			lock the target subdirectory
	else
		if source is a subdirectory
			lock the source
		if target is a subdirectory
			lock the target
	lock non-directories involved, in inode pointer order if both
	source and target are such.

That way we are guaranteed that parents are locked (for obvious reasons),
that any renamed non-directory is locked (nfsd relies upon that),
that any victim is locked (emptiness check needs that, among other things)
and subdirectory that changes parent is locked (needed to protect the update
of .. entries).  We are also guaranteed that any operation locking more
than one directory either takes ->s_vfs_rename_mutex or locks a parent
followed by its child.

Cc: stable@vger.kernel.org
Fixes: 28eceeda130f "fs: Lock moved directories"
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 15 insertions, 14 deletions

diff --git a/Documentation/filesystems/directory-locking.rst b/Documentation/filesystems/directory-locking.rst
index dccd61c7c5c3..193c22687851 100644
--- a/Documentation/filesystems/directory-locking.rst
+++ b/Documentation/filesystems/directory-locking.rst
@@ -22,13 +22,16 @@ exclusive.
 3) object removal.  Locking rules: caller locks parent, finds victim,
 locks victim and calls the method.  Locks are exclusive.
 
-4) rename() that is _not_ cross-directory.  Locking rules: caller locks the
-parent and finds source and target.  We lock both (provided they exist).  If we
-need to lock two inodes of different type (dir vs non-dir), we lock directory
-first.  If we need to lock two inodes of the same type, lock them in inode
-pointer order.  Then call the method.  All locks are exclusive.
-NB: we might get away with locking the source (and target in exchange
-case) shared.
+4) rename() that is _not_ cross-directory.  Locking rules: caller locks
+the parent and finds source and target.  Then we decide which of the
+source and target need to be locked.  Source needs to be locked if it's a
+non-directory; target - if it's a non-directory or about to be removed.
+Take the locks that need to be taken, in inode pointer order if need
+to take both (that can happen only when both source and target are
+non-directories - the source because it wouldn't be locked otherwise
+and the target because mixing directory and non-directory is allowed
+only with RENAME_EXCHANGE, and that won't be removing the target).
+After the locks had been taken, call the method.  All locks are exclusive.
 
 5) link creation.  Locking rules:
 
@@ -44,20 +47,17 @@ rules:
 
 	* lock the filesystem
 	* lock parents in "ancestors first" order. If one is not ancestor of
-	  the other, lock them in inode pointer order.
+	  the other, lock the parent of source first.
 	* find source and target.
 	* if old parent is equal to or is a descendent of target
 	  fail with -ENOTEMPTY
 	* if new parent is equal to or is a descendent of source
 	  fail with -ELOOP
-	* Lock both the source and the target provided they exist. If we
-	  need to lock two inodes of different type (dir vs non-dir), we lock
-	  the directory first. If we need to lock two inodes of the same type,
-	  lock them in inode pointer order.
+	* Lock subdirectories involved (source before target).
+	* Lock non-directories involved, in inode pointer order.
 	* call the method.
 
-All ->i_rwsem are taken exclusive.  Again, we might get away with locking
-the source (and target in exchange case) shared.
+All ->i_rwsem are taken exclusive.
 
 The rules above obviously guarantee that all directories that are going to be
 read, modified or removed by method will be locked by caller.
@@ -67,6 +67,7 @@ If no directory is its own ancestor, the scheme above is deadlock-free.
 
 Proof:
 
+[XXX: will be updated once we are done massaging the lock_rename()]
 	First of all, at any moment we have a linear ordering of the
 	objects - A < B iff (A is an ancestor of B) or (B is not an ancestor
         of A and ptr(A) < ptr(B)).