linux/Documentation/filesystems/directory-locking.rst
<<
>>
Prefs
   1=================
   2Directory Locking
   3=================
   4
   5
   6Locking scheme used for directory operations is based on two
   7kinds of locks - per-inode (->i_rwsem) and per-filesystem
   8(->s_vfs_rename_mutex).
   9
  10When taking the i_rwsem on multiple non-directory objects, we
  11always acquire the locks in order by increasing address.  We'll call
  12that "inode pointer" order in the following.
  13
  14For our purposes all operations fall in 5 classes:
  15
  161) read access.  Locking rules: caller locks directory we are accessing.
  17The lock is taken shared.
  18
  192) object creation.  Locking rules: same as above, but the lock is taken
  20exclusive.
  21
  223) object removal.  Locking rules: caller locks parent, finds victim,
  23locks victim and calls the method.  Locks are exclusive.
  24
  254) rename() that is _not_ cross-directory.  Locking rules: caller locks
  26the parent and finds source and target.  In case of exchange (with
  27RENAME_EXCHANGE in flags argument) lock both.  In any case,
  28if the target already exists, lock it.  If the source is a non-directory,
  29lock it.  If we need to lock both, lock them in inode pointer order.
  30Then call the method.  All locks are exclusive.
  31NB: we might get away with locking the source (and target in exchange
  32case) shared.
  33
  345) link creation.  Locking rules:
  35
  36        * lock parent
  37        * check that source is not a directory
  38        * lock source
  39        * call the method.
  40
  41All locks are exclusive.
  42
  436) cross-directory rename.  The trickiest in the whole bunch.  Locking
  44rules:
  45
  46        * lock the filesystem
  47        * lock parents in "ancestors first" order.
  48        * find source and target.
  49        * if old parent is equal to or is a descendent of target
  50          fail with -ENOTEMPTY
  51        * if new parent is equal to or is a descendent of source
  52          fail with -ELOOP
  53        * If it's an exchange, lock both the source and the target.
  54        * If the target exists, lock it.  If the source is a non-directory,
  55          lock it.  If we need to lock both, do so in inode pointer order.
  56        * call the method.
  57
  58All ->i_rwsem are taken exclusive.  Again, we might get away with locking
  59the source (and target in exchange case) shared.
  60
  61The rules above obviously guarantee that all directories that are going to be
  62read, modified or removed by method will be locked by caller.
  63
  64
  65If no directory is its own ancestor, the scheme above is deadlock-free.
  66
  67Proof:
  68
  69        First of all, at any moment we have a partial ordering of the
  70        objects - A < B iff A is an ancestor of B.
  71
  72        That ordering can change.  However, the following is true:
  73
  74(1) if object removal or non-cross-directory rename holds lock on A and
  75    attempts to acquire lock on B, A will remain the parent of B until we
  76    acquire the lock on B.  (Proof: only cross-directory rename can change
  77    the parent of object and it would have to lock the parent).
  78
  79(2) if cross-directory rename holds the lock on filesystem, order will not
  80    change until rename acquires all locks.  (Proof: other cross-directory
  81    renames will be blocked on filesystem lock and we don't start changing
  82    the order until we had acquired all locks).
  83
  84(3) locks on non-directory objects are acquired only after locks on
  85    directory objects, and are acquired in inode pointer order.
  86    (Proof: all operations but renames take lock on at most one
  87    non-directory object, except renames, which take locks on source and
  88    target in inode pointer order in the case they are not directories.)
  89
  90Now consider the minimal deadlock.  Each process is blocked on
  91attempt to acquire some lock and already holds at least one lock.  Let's
  92consider the set of contended locks.  First of all, filesystem lock is
  93not contended, since any process blocked on it is not holding any locks.
  94Thus all processes are blocked on ->i_rwsem.
  95
  96By (3), any process holding a non-directory lock can only be
  97waiting on another non-directory lock with a larger address.  Therefore
  98the process holding the "largest" such lock can always make progress, and
  99non-directory objects are not included in the set of contended locks.
 100
 101Thus link creation can't be a part of deadlock - it can't be
 102blocked on source and it means that it doesn't hold any locks.
 103
 104Any contended object is either held by cross-directory rename or
 105has a child that is also contended.  Indeed, suppose that it is held by
 106operation other than cross-directory rename.  Then the lock this operation
 107is blocked on belongs to child of that object due to (1).
 108
 109It means that one of the operations is cross-directory rename.
 110Otherwise the set of contended objects would be infinite - each of them
 111would have a contended child and we had assumed that no object is its
 112own descendent.  Moreover, there is exactly one cross-directory rename
 113(see above).
 114
 115Consider the object blocking the cross-directory rename.  One
 116of its descendents is locked by cross-directory rename (otherwise we
 117would again have an infinite set of contended objects).  But that
 118means that cross-directory rename is taking locks out of order.  Due
 119to (2) the order hadn't changed since we had acquired filesystem lock.
 120But locking rules for cross-directory rename guarantee that we do not
 121try to acquire lock on descendent before the lock on ancestor.
 122Contradiction.  I.e.  deadlock is impossible.  Q.E.D.
 123
 124
 125These operations are guaranteed to avoid loop creation.  Indeed,
 126the only operation that could introduce loops is cross-directory rename.
 127Since the only new (parent, child) pair added by rename() is (new parent,
 128source), such loop would have to contain these objects and the rest of it
 129would have to exist before rename().  I.e. at the moment of loop creation
 130rename() responsible for that would be holding filesystem lock and new parent
 131would have to be equal to or a descendent of source.  But that means that
 132new parent had been equal to or a descendent of source since the moment when
 133we had acquired filesystem lock and rename() would fail with -ELOOP in that
 134case.
 135
 136While this locking scheme works for arbitrary DAGs, it relies on
 137ability to check that directory is a descendent of another object.  Current
 138implementation assumes that directory graph is a tree.  This assumption is
 139also preserved by all operations (cross-directory rename on a tree that would
 140not introduce a cycle will leave it a tree and link() fails for directories).
 141
 142Notice that "directory" in the above == "anything that might have
 143children", so if we are going to introduce hybrid objects we will need
 144either to make sure that link(2) doesn't work for them or to make changes
 145in is_subdir() that would make it work even in presence of such beasts.
 146