About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / filesystems / directory-locking




Custom Search

Based on kernel version 4.7.2. Page generated on 2016-08-22 22:45 EST.

1		Locking scheme used for directory operations is based on two
2	kinds of locks - per-inode (->i_rwsem) and per-filesystem
3	(->s_vfs_rename_mutex).
4	
5		When taking the i_rwsem on multiple non-directory objects, we
6	always acquire the locks in order by increasing address.  We'll call
7	that "inode pointer" order in the following.
8	
9		For our purposes all operations fall in 5 classes:
10	
11	1) read access.  Locking rules: caller locks directory we are accessing.
12	The lock is taken shared.
13	
14	2) object creation.  Locking rules: same as above, but the lock is taken
15	exclusive.
16	
17	3) object removal.  Locking rules: caller locks parent, finds victim,
18	locks victim and calls the method.  Locks are exclusive.
19	
20	4) rename() that is _not_ cross-directory.  Locking rules: caller locks
21	the parent and finds source and target.  In case of exchange (with
22	RENAME_EXCHANGE in rename2() flags argument) lock both.  In any case,
23	if the target already exists, lock it.  If the source is a non-directory,
24	lock it.  If we need to lock both, lock them in inode pointer order.
25	Then call the method.  All locks are exclusive.
26	NB: we might get away with locking the the source (and target in exchange
27	case) shared.
28	
29	5) link creation.  Locking rules:
30		* lock parent
31		* check that source is not a directory
32		* lock source
33		* call the method.
34	All locks are exclusive.
35	
36	6) cross-directory rename.  The trickiest in the whole bunch.  Locking
37	rules:
38		* lock the filesystem
39		* lock parents in "ancestors first" order.
40		* find source and target.
41		* if old parent is equal to or is a descendent of target
42			fail with -ENOTEMPTY
43		* if new parent is equal to or is a descendent of source
44			fail with -ELOOP
45		* If it's an exchange, lock both the source and the target.
46		* If the target exists, lock it.  If the source is a non-directory,
47		  lock it.  If we need to lock both, do so in inode pointer order.
48		* call the method.
49	All ->i_rwsem are taken exclusive.  Again, we might get away with locking
50	the the source (and target in exchange case) shared.
51	
52	The rules above obviously guarantee that all directories that are going to be
53	read, modified or removed by method will be locked by caller.
54	
55	
56	If no directory is its own ancestor, the scheme above is deadlock-free.
57	Proof:
58	
59		First of all, at any moment we have a partial ordering of the
60	objects - A < B iff A is an ancestor of B.
61	
62		That ordering can change.  However, the following is true:
63	
64	(1) if object removal or non-cross-directory rename holds lock on A and
65	    attempts to acquire lock on B, A will remain the parent of B until we
66	    acquire the lock on B.  (Proof: only cross-directory rename can change
67	    the parent of object and it would have to lock the parent).
68	
69	(2) if cross-directory rename holds the lock on filesystem, order will not
70	    change until rename acquires all locks.  (Proof: other cross-directory
71	    renames will be blocked on filesystem lock and we don't start changing
72	    the order until we had acquired all locks).
73	
74	(3) locks on non-directory objects are acquired only after locks on
75	    directory objects, and are acquired in inode pointer order.
76	    (Proof: all operations but renames take lock on at most one
77	    non-directory object, except renames, which take locks on source and
78	    target in inode pointer order in the case they are not directories.)
79	
80		Now consider the minimal deadlock.  Each process is blocked on
81	attempt to acquire some lock and already holds at least one lock.  Let's
82	consider the set of contended locks.  First of all, filesystem lock is
83	not contended, since any process blocked on it is not holding any locks.
84	Thus all processes are blocked on ->i_rwsem.
85	
86		By (3), any process holding a non-directory lock can only be
87	waiting on another non-directory lock with a larger address.  Therefore
88	the process holding the "largest" such lock can always make progress, and
89	non-directory objects are not included in the set of contended locks.
90	
91		Thus link creation can't be a part of deadlock - it can't be
92	blocked on source and it means that it doesn't hold any locks.
93	
94		Any contended object is either held by cross-directory rename or
95	has a child that is also contended.  Indeed, suppose that it is held by
96	operation other than cross-directory rename.  Then the lock this operation
97	is blocked on belongs to child of that object due to (1).
98	
99		It means that one of the operations is cross-directory rename.
100	Otherwise the set of contended objects would be infinite - each of them
101	would have a contended child and we had assumed that no object is its
102	own descendent.  Moreover, there is exactly one cross-directory rename
103	(see above).
104	
105		Consider the object blocking the cross-directory rename.  One
106	of its descendents is locked by cross-directory rename (otherwise we
107	would again have an infinite set of contended objects).  But that
108	means that cross-directory rename is taking locks out of order.  Due
109	to (2) the order hadn't changed since we had acquired filesystem lock.
110	But locking rules for cross-directory rename guarantee that we do not
111	try to acquire lock on descendent before the lock on ancestor.
112	Contradiction.  I.e.  deadlock is impossible.  Q.E.D.
113	
114	
115		These operations are guaranteed to avoid loop creation.  Indeed,
116	the only operation that could introduce loops is cross-directory rename.
117	Since the only new (parent, child) pair added by rename() is (new parent,
118	source), such loop would have to contain these objects and the rest of it
119	would have to exist before rename().  I.e. at the moment of loop creation
120	rename() responsible for that would be holding filesystem lock and new parent
121	would have to be equal to or a descendent of source.  But that means that
122	new parent had been equal to or a descendent of source since the moment when
123	we had acquired filesystem lock and rename() would fail with -ELOOP in that
124	case.
125	
126		While this locking scheme works for arbitrary DAGs, it relies on
127	ability to check that directory is a descendent of another object.  Current
128	implementation assumes that directory graph is a tree.  This assumption is
129	also preserved by all operations (cross-directory rename on a tree that would
130	not introduce a cycle will leave it a tree and link() fails for directories).
131	
132		Notice that "directory" in the above == "anything that might have
133	children", so if we are going to introduce hybrid objects we will need
134	either to make sure that link(2) doesn't work for them or to make changes
135	in is_subdir() that would make it work even in presence of such beasts.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.