rename(): avoid a deadlock in the case of parents having no common ancestor

... and fix the directory locking documentation and proof of correctness.
Holding ->s_vfs_rename_mutex *almost* prevents ->d_parent changes; the
case where we really don't want it is splicing the root of disconnected
tree to somewhere.

In other words, ->s_vfs_rename_mutex is sufficient to stabilize "X is an
ancestor of Y" only if X and Y are already in the same tree.  Otherwise
it can go from false to true, and one can construct a deadlock on that.

Make lock_two_directories() report an error in such case and update the
callers of lock_rename()/lock_rename_child() to handle such errors.

And yes, such conditions are not impossible to create ;-/

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 242 insertions, 104 deletions

diff --git a/Documentation/filesystems/directory-locking.rst b/Documentation/filesystems/directory-locking.rst
index 193c22687851..05ea387bc9fb 100644
--- a/Documentation/filesystems/directory-locking.rst
+++ b/Documentation/filesystems/directory-locking.rst
@@ -11,130 +11,268 @@ When taking the i_rwsem on multiple non-directory objects, we
 always acquire the locks in order by increasing address.  We'll call
 that "inode pointer" order in the following.
 
-For our purposes all operations fall in 5 classes:
 
-1) read access.  Locking rules: caller locks directory we are accessing.
-The lock is taken shared.
+Primitives
+==========
 
-2) object creation.  Locking rules: same as above, but the lock is taken
-exclusive.
+For our purposes all operations fall in 6 classes:
 
-3) object removal.  Locking rules: caller locks parent, finds victim,
-locks victim and calls the method.  Locks are exclusive.
+1. read access.  Locking rules:
 
-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.
+	* lock the directory we are accessing (shared)
 
-5) link creation.  Locking rules:
+2. object creation.  Locking rules:
 
-	* lock parent
-	* check that source is not a directory
-	* lock source
-	* call the method.
+	* lock the directory we are accessing (exclusive)
 
-All locks are exclusive.
+3. object removal.  Locking rules:
 
-6) cross-directory rename.  The trickiest in the whole bunch.  Locking
-rules:
+	* lock the parent (exclusive)
+	* find the victim
+	* lock the victim (exclusive)
 
-	* lock the filesystem
-	* lock parents in "ancestors first" order. If one is not ancestor of
-	  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 subdirectories involved (source before target).
-	* Lock non-directories involved, in inode pointer order.
-	* call the method.
+4. link creation.  Locking rules:
+
+	* lock the parent (exclusive)
+	* check that the source is not a directory
+	* lock the source (exclusive; probably could be weakened to shared)
 
-All ->i_rwsem are taken exclusive.
+5. rename that is _not_ cross-directory.  Locking rules:
 
-The rules above obviously guarantee that all directories that are going to be
-read, modified or removed by method will be locked by caller.
+	* lock the parent (exclusive)
+	* find the source and target
+	* decide which of the source and target need to be locked.
+	  The 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 (exlusive), 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 need to 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).
 
+6. cross-directory rename.  The trickiest in the whole bunch.  Locking rules:
+
+	* lock the filesystem
+	* if the parents don't have a common ancestor, fail the operation.
+	* lock the parents in "ancestors first" order (exclusive). If neither is an
+	  ancestor of the other, lock the parent of source first.
+	* find the source and target.
+	* verify that the source is not a descendent of the target and
+	  target is not a descendent of source; fail the operation otherwise.
+	* lock the subdirectories involved (exclusive), source before target.
+	* lock the non-directories involved (exclusive), in inode pointer order.
+
+The rules above obviously guarantee that all directories that are going
+to be read, modified or removed by method will be locked by the caller.
+
+
+Splicing
+========
+
+There is one more thing to consider - splicing.  It's not an operation
+in its own right; it may happen as part of lookup.  We speak of the
+operations on directory trees, but we obviously do not have the full
+picture of those - especially for network filesystems.  What we have
+is a bunch of subtrees visible in dcache and locking happens on those.
+Trees grow as we do operations; memory pressure prunes them.  Normally
+that's not a problem, but there is a nasty twist - what should we do
+when one growing tree reaches the root of another?  That can happen in
+several scenarios, starting from "somebody mounted two nested subtrees
+from the same NFS4 server and doing lookups in one of them has reached
+the root of another"; there's also open-by-fhandle stuff, and there's a
+possibility that directory we see in one place gets moved by the server
+to another and we run into it when we do a lookup.
+
+For a lot of reasons we want to have the same directory present in dcache
+only once.  Multiple aliases are not allowed.  So when lookup runs into
+a subdirectory that already has an alias, something needs to be done with
+dcache trees.  Lookup is already holding the parent locked.  If alias is
+a root of separate tree, it gets attached to the directory we are doing a
+lookup in, under the name we'd been looking for.  If the alias is already
+a child of the directory we are looking in, it changes name to the one
+we'd been looking for.  No extra locking is involved in these two cases.
+However, if it's a child of some other directory, the things get trickier.
+First of all, we verify that it is *not* an ancestor of our directory
+and fail the lookup if it is.  Then we try to lock the filesystem and the
+current parent of the alias.  If either trylock fails, we fail the lookup.
+If trylocks succeed, we detach the alias from its current parent and
+attach to our directory, under the name we are looking for.
+
+Note that splicing does *not* involve any modification of the filesystem;
+all we change is the view in dcache.  Moreover, holding a directory locked
+exclusive prevents such changes involving its children and holding the
+filesystem lock prevents any changes of tree topology, other than having a
+root of one tree becoming a child of directory in another.  In particular,
+if two dentries have been found to have a common ancestor after taking
+the filesystem lock, their relationship will remain unchanged until
+the lock is dropped.  So from the directory operations' point of view
+splicing is almost irrelevant - the only place where it matters is one
+step in cross-directory renames; we need to be careful when checking if
+parents have a common ancestor.
+
+
+Multiple-filesystem stuff
+=========================
+
+For some filesystems a method can involve a directory operation on
+another filesystem; it may be ecryptfs doing operation in the underlying
+filesystem, overlayfs doing something to the layers, network filesystem
+using a local one as a cache, etc.  In all such cases the operations
+on other filesystems must follow the same locking rules.  Moreover, "a
+directory operation on this filesystem might involve directory operations
+on that filesystem" should be an asymmetric relation (or, if you will,
+it should be possible to rank the filesystems so that directory operation
+on a filesystem could trigger directory operations only on higher-ranked
+ones - in these terms overlayfs ranks lower than its layers, network
+filesystem ranks lower than whatever it caches on, etc.)
+
+
+Deadlock avoidance
+==================
 
 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)).
-
-	That ordering can change.  However, the following is true:
-
-(1) if object removal or non-cross-directory rename holds lock on A and
-    attempts to acquire lock on B, A will remain the parent of B until we
-    acquire the lock on B.  (Proof: only cross-directory rename can change
-    the parent of object and it would have to lock the parent).
-
-(2) if cross-directory rename holds the lock on filesystem, order will not
-    change until rename acquires all locks.  (Proof: other cross-directory
-    renames will be blocked on filesystem lock and we don't start changing
-    the order until we had acquired all locks).
-
-(3) locks on non-directory objects are acquired only after locks on
-    directory objects, and are acquired in inode pointer order.
-    (Proof: all operations but renames take lock on at most one
-    non-directory object, except renames, which take locks on source and
-    target in inode pointer order in the case they are not directories.)
-
-Now consider the minimal deadlock.  Each process is blocked on
-attempt to acquire some lock and already holds at least one lock.  Let's
-consider the set of contended locks.  First of all, filesystem lock is
-not contended, since any process blocked on it is not holding any locks.
-Thus all processes are blocked on ->i_rwsem.
-
-By (3), any process holding a non-directory lock can only be
-waiting on another non-directory lock with a larger address.  Therefore
-the process holding the "largest" such lock can always make progress, and
-non-directory objects are not included in the set of contended locks.
-
-Thus link creation can't be a part of deadlock - it can't be
-blocked on source and it means that it doesn't hold any locks.
-
-Any contended object is either held by cross-directory rename or
-has a child that is also contended.  Indeed, suppose that it is held by
-operation other than cross-directory rename.  Then the lock this operation
-is blocked on belongs to child of that object due to (1).
-
-It means that one of the operations is cross-directory rename.
-Otherwise the set of contended objects would be infinite - each of them
-would have a contended child and we had assumed that no object is its
-own descendent.  Moreover, there is exactly one cross-directory rename
-(see above).
-
-Consider the object blocking the cross-directory rename.  One
-of its descendents is locked by cross-directory rename (otherwise we
-would again have an infinite set of contended objects).  But that
-means that cross-directory rename is taking locks out of order.  Due
-to (2) the order hadn't changed since we had acquired filesystem lock.
-But locking rules for cross-directory rename guarantee that we do not
-try to acquire lock on descendent before the lock on ancestor.
-Contradiction.  I.e.  deadlock is impossible.  Q.E.D.
-
+There is a ranking on the locks, such that all primitives take
+them in order of non-decreasing rank.  Namely,
+
+  * rank ->i_rwsem of non-directories on given filesystem in inode pointer
+    order.
+  * put ->i_rwsem of all directories on a filesystem at the same rank,
+    lower than ->i_rwsem of any non-directory on the same filesystem.
+  * put ->s_vfs_rename_mutex at rank lower than that of any ->i_rwsem
+    on the same filesystem.
+  * among the locks on different filesystems use the relative
+    rank of those filesystems.
+
+For example, if we have NFS filesystem caching on a local one, we have
+
+  1. ->s_vfs_rename_mutex of NFS filesystem
+  2. ->i_rwsem of directories on that NFS filesystem, same rank for all
+  3. ->i_rwsem of non-directories on that filesystem, in order of
+     increasing address of inode
+  4. ->s_vfs_rename_mutex of local filesystem
+  5. ->i_rwsem of directories on the local filesystem, same rank for all
+  6. ->i_rwsem of non-directories on local filesystem, in order of
+     increasing address of inode.
+
+It's easy to verify that operations never take a lock with rank
+lower than that of an already held lock.
+
+Suppose deadlocks are possible.  Consider the minimal deadlocked
+set of threads.  It is a cycle of several threads, each blocked on a lock
+held by the next thread in the cycle.
+
+Since the locking order is consistent with the ranking, all
+contended locks in the minimal deadlock will be of the same rank,
+i.e. they all will be ->i_rwsem of directories on the same filesystem.
+Moreover, without loss of generality we can assume that all operations
+are done directly to that filesystem and none of them has actually
+reached the method call.
+
+In other words, we have a cycle of threads, T1,..., Tn,
+and the same number of directories (D1,...,Dn) such that
+
+	T1 is blocked on D1 which is held by T2
+
+	T2 is blocked on D2 which is held by T3
+
+	...
+
+	Tn is blocked on Dn which is held by T1.
+
+Each operation in the minimal cycle must have locked at least
+one directory and blocked on attempt to lock another.  That leaves
+only 3 possible operations: directory removal (locks parent, then
+child), same-directory rename killing a subdirectory (ditto) and
+cross-directory rename of some sort.
+
+There must be a cross-directory rename in the set; indeed,
+if all operations had been of the "lock parent, then child" sort
+we would have Dn a parent of D1, which is a parent of D2, which is
+a parent of D3, ..., which is a parent of Dn.  Relationships couldn't
+have changed since the moment directory locks had been acquired,
+so they would all hold simultaneously at the deadlock time and
+we would have a loop.
+
+Since all operations are on the same filesystem, there can't be
+more than one cross-directory rename among them.  Without loss of
+generality we can assume that T1 is the one doing a cross-directory
+rename and everything else is of the "lock parent, then child" sort.
+
+In other words, we have a cross-directory rename that locked
+Dn and blocked on attempt to lock D1, which is a parent of D2, which is
+a parent of D3, ..., which is a parent of Dn.  Relationships between
+D1,...,Dn all hold simultaneously at the deadlock time.  Moreover,
+cross-directory rename does not get to locking any directories until it
+has acquired filesystem lock and verified that directories involved have
+a common ancestor, which guarantees that ancestry relationships between
+all of them had been stable.
+
+Consider the order in which directories are locked by the
+cross-directory rename; parents first, then possibly their children.
+Dn and D1 would have to be among those, with Dn locked before D1.
+Which pair could it be?
+
+It can't be the parents - indeed, since D1 is an ancestor of Dn,
+it would be the first parent to be locked.  Therefore at least one of the
+children must be involved and thus neither of them could be a descendent
+of another - otherwise the operation would not have progressed past
+locking the parents.
+
+It can't be a parent and its child; otherwise we would've had
+a loop, since the parents are locked before the children, so the parent
+would have to be a descendent of its child.
+
+It can't be a parent and a child of another parent either.
+Otherwise the child of the parent in question would've been a descendent
+of another child.
+
+That leaves only one possibility - namely, both Dn and D1 are
+among the children, in some order.  But that is also impossible, since
+neither of the children is a descendent of another.
+
+That concludes the proof, since the set of operations with the
+properties requiered for a minimal deadlock can not exist.
+
+Note that the check for having a common ancestor in cross-directory
+rename is crucial - without it a deadlock would be possible.  Indeed,
+suppose the parents are initially in different trees; we would lock the
+parent of source, then try to lock the parent of target, only to have
+an unrelated lookup splice a distant ancestor of source to some distant
+descendent of the parent of target.   At that point we have cross-directory
+rename holding the lock on parent of source and trying to lock its
+distant ancestor.  Add a bunch of rmdir() attempts on all directories
+in between (all of those would fail with -ENOTEMPTY, had they ever gotten
+the locks) and voila - we have a deadlock.
+
+Loop avoidance
+==============
 
 These operations are guaranteed to avoid loop creation.  Indeed,
 the only operation that could introduce loops is cross-directory rename.
-Since the only new (parent, child) pair added by rename() is (new parent,
-source), such loop would have to contain these objects and the rest of it
-would have to exist before rename().  I.e. at the moment of loop creation
-rename() responsible for that would be holding filesystem lock and new parent
-would have to be equal to or a descendent of source.  But that means that
-new parent had been equal to or a descendent of source since the moment when
-we had acquired filesystem lock and rename() would fail with -ELOOP in that
-case.
+Suppose after the operation there is a loop; since there hadn't been such
+loops before the operation, at least on of the nodes in that loop must've
+had its parent changed.  In other words, the loop must be passing through
+the source or, in case of exchange, possibly the target.
+
+Since the operation has succeeded, neither source nor target could have
+been ancestors of each other.  Therefore the chain of ancestors starting
+in the parent of source could not have passed through the target and
+vice versa.  On the other hand, the chain of ancestors of any node could
+not have passed through the node itself, or we would've had a loop before
+the operation.  But everything other than source and target has kept
+the parent after the operation, so the operation does not change the
+chains of ancestors of (ex-)parents of source and target.  In particular,
+those chains must end after a finite number of steps.
+
+Now consider the loop created by the operation.  It passes through either
+source or target; the next node in the loop would be the ex-parent of
+target or source resp.  After that the loop would follow the chain of
+ancestors of that parent.  But as we have just shown, that chain must
+end after a finite number of steps, which means that it can't be a part
+of any loop.  Q.E.D.
 
 While this locking scheme works for arbitrary DAGs, it relies on
 ability to check that directory is a descendent of another object.  Current