Based on kernel version 4.2. Page generated on 2015-09-09 12:13 EST.
1 Locking scheme used for directory operations is based on two 2 kinds of locks - per-inode (->i_mutex) and per-filesystem 3 (->s_vfs_rename_mutex). 4 5 When taking the i_mutex 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 13 2) object creation. Locking rules: same as above. 14 15 3) object removal. Locking rules: caller locks parent, finds victim, 16 locks victim and calls the method. 17 18 4) rename() that is _not_ cross-directory. Locking rules: caller locks 19 the parent and finds source and target. If target already exists, lock 20 it. If source is a non-directory, lock it. If that means we need to 21 lock both, lock them in inode pointer order. 22 23 5) link creation. Locking rules: 24 * lock parent 25 * check that source is not a directory 26 * lock source 27 * call the method. 28 29 6) cross-directory rename. The trickiest in the whole bunch. Locking 30 rules: 31 * lock the filesystem 32 * lock parents in "ancestors first" order. 33 * find source and target. 34 * if old parent is equal to or is a descendent of target 35 fail with -ENOTEMPTY 36 * if new parent is equal to or is a descendent of source 37 fail with -ELOOP 38 * If target exists, lock it. If source is a non-directory, lock 39 it. In case that means we need to lock both source and target, 40 do so in inode pointer order. 41 * call the method. 42 43 44 The rules above obviously guarantee that all directories that are going to be 45 read, modified or removed by method will be locked by caller. 46 47 48 If no directory is its own ancestor, the scheme above is deadlock-free. 49 Proof: 50 51 First of all, at any moment we have a partial ordering of the 52 objects - A < B iff A is an ancestor of B. 53 54 That ordering can change. However, the following is true: 55 56 (1) if object removal or non-cross-directory rename holds lock on A and 57 attempts to acquire lock on B, A will remain the parent of B until we 58 acquire the lock on B. (Proof: only cross-directory rename can change 59 the parent of object and it would have to lock the parent). 60 61 (2) if cross-directory rename holds the lock on filesystem, order will not 62 change until rename acquires all locks. (Proof: other cross-directory 63 renames will be blocked on filesystem lock and we don't start changing 64 the order until we had acquired all locks). 65 66 (3) locks on non-directory objects are acquired only after locks on 67 directory objects, and are acquired in inode pointer order. 68 (Proof: all operations but renames take lock on at most one 69 non-directory object, except renames, which take locks on source and 70 target in inode pointer order in the case they are not directories.) 71 72 Now consider the minimal deadlock. Each process is blocked on 73 attempt to acquire some lock and already holds at least one lock. Let's 74 consider the set of contended locks. First of all, filesystem lock is 75 not contended, since any process blocked on it is not holding any locks. 76 Thus all processes are blocked on ->i_mutex. 77 78 By (3), any process holding a non-directory lock can only be 79 waiting on another non-directory lock with a larger address. Therefore 80 the process holding the "largest" such lock can always make progress, and 81 non-directory objects are not included in the set of contended locks. 82 83 Thus link creation can't be a part of deadlock - it can't be 84 blocked on source and it means that it doesn't hold any locks. 85 86 Any contended object is either held by cross-directory rename or 87 has a child that is also contended. Indeed, suppose that it is held by 88 operation other than cross-directory rename. Then the lock this operation 89 is blocked on belongs to child of that object due to (1). 90 91 It means that one of the operations is cross-directory rename. 92 Otherwise the set of contended objects would be infinite - each of them 93 would have a contended child and we had assumed that no object is its 94 own descendent. Moreover, there is exactly one cross-directory rename 95 (see above). 96 97 Consider the object blocking the cross-directory rename. One 98 of its descendents is locked by cross-directory rename (otherwise we 99 would again have an infinite set of contended objects). But that 100 means that cross-directory rename is taking locks out of order. Due 101 to (2) the order hadn't changed since we had acquired filesystem lock. 102 But locking rules for cross-directory rename guarantee that we do not 103 try to acquire lock on descendent before the lock on ancestor. 104 Contradiction. I.e. deadlock is impossible. Q.E.D. 105 106 107 These operations are guaranteed to avoid loop creation. Indeed, 108 the only operation that could introduce loops is cross-directory rename. 109 Since the only new (parent, child) pair added by rename() is (new parent, 110 source), such loop would have to contain these objects and the rest of it 111 would have to exist before rename(). I.e. at the moment of loop creation 112 rename() responsible for that would be holding filesystem lock and new parent 113 would have to be equal to or a descendent of source. But that means that 114 new parent had been equal to or a descendent of source since the moment when 115 we had acquired filesystem lock and rename() would fail with -ELOOP in that 116 case. 117 118 While this locking scheme works for arbitrary DAGs, it relies on 119 ability to check that directory is a descendent of another object. Current 120 implementation assumes that directory graph is a tree. This assumption is 121 also preserved by all operations (cross-directory rename on a tree that would 122 not introduce a cycle will leave it a tree and link() fails for directories). 123 124 Notice that "directory" in the above == "anything that might have 125 children", so if we are going to introduce hybrid objects we will need 126 either to make sure that link(2) doesn't work for them or to make changes 127 in is_subdir() that would make it work even in presence of such beasts.