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 the 26parent and finds source and target. We lock both (provided they exist). If we 27need to lock two inodes of different type (dir vs non-dir), we lock directory 28first. If we need to lock two inodes of the same type, lock them in inode 29pointer order. Then call the method. All locks are exclusive. 30NB: we might get away with locking the source (and target in exchange 31case) shared. 32 335) link creation. Locking rules: 34 35 * lock parent 36 * check that source is not a directory 37 * lock source 38 * call the method. 39 40All locks are exclusive. 41 426) cross-directory rename. The trickiest in the whole bunch. Locking 43rules: 44 45 * lock the filesystem 46 * lock parents in "ancestors first" order. If one is not ancestor of 47 the other, lock them in inode pointer 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 * Lock both the source and the target provided they exist. If we 54 need to lock two inodes of different type (dir vs non-dir), we lock 55 the directory first. If we need to lock two inodes of the same type, 56 lock them in inode pointer order. 57 * call the method. 58 59All ->i_rwsem are taken exclusive. Again, we might get away with locking 60the source (and target in exchange case) shared. 61 62The rules above obviously guarantee that all directories that are going to be 63read, modified or removed by method will be locked by caller. 64 65 66If no directory is its own ancestor, the scheme above is deadlock-free. 67 68Proof: 69 70 First of all, at any moment we have a linear ordering of the 71 objects - A < B iff (A is an ancestor of B) or (B is not an ancestor 72 of A and ptr(A) < ptr(B)). 73 74 That ordering can change. However, the following is true: 75 76(1) if object removal or non-cross-directory rename holds lock on A and 77 attempts to acquire lock on B, A will remain the parent of B until we 78 acquire the lock on B. (Proof: only cross-directory rename can change 79 the parent of object and it would have to lock the parent). 80 81(2) if cross-directory rename holds the lock on filesystem, order will not 82 change until rename acquires all locks. (Proof: other cross-directory 83 renames will be blocked on filesystem lock and we don't start changing 84 the order until we had acquired all locks). 85 86(3) locks on non-directory objects are acquired only after locks on 87 directory objects, and are acquired in inode pointer order. 88 (Proof: all operations but renames take lock on at most one 89 non-directory object, except renames, which take locks on source and 90 target in inode pointer order in the case they are not directories.) 91 92Now consider the minimal deadlock. Each process is blocked on 93attempt to acquire some lock and already holds at least one lock. Let's 94consider the set of contended locks. First of all, filesystem lock is 95not contended, since any process blocked on it is not holding any locks. 96Thus all processes are blocked on ->i_rwsem. 97 98By (3), any process holding a non-directory lock can only be 99waiting on another non-directory lock with a larger address. Therefore 100the process holding the "largest" such lock can always make progress, and 101non-directory objects are not included in the set of contended locks. 102 103Thus link creation can't be a part of deadlock - it can't be 104blocked on source and it means that it doesn't hold any locks. 105 106Any contended object is either held by cross-directory rename or 107has a child that is also contended. Indeed, suppose that it is held by 108operation other than cross-directory rename. Then the lock this operation 109is blocked on belongs to child of that object due to (1). 110 111It means that one of the operations is cross-directory rename. 112Otherwise the set of contended objects would be infinite - each of them 113would have a contended child and we had assumed that no object is its 114own descendent. Moreover, there is exactly one cross-directory rename 115(see above). 116 117Consider the object blocking the cross-directory rename. One 118of its descendents is locked by cross-directory rename (otherwise we 119would again have an infinite set of contended objects). But that 120means that cross-directory rename is taking locks out of order. Due 121to (2) the order hadn't changed since we had acquired filesystem lock. 122But locking rules for cross-directory rename guarantee that we do not 123try to acquire lock on descendent before the lock on ancestor. 124Contradiction. I.e. deadlock is impossible. Q.E.D. 125 126 127These operations are guaranteed to avoid loop creation. Indeed, 128the only operation that could introduce loops is cross-directory rename. 129Since the only new (parent, child) pair added by rename() is (new parent, 130source), such loop would have to contain these objects and the rest of it 131would have to exist before rename(). I.e. at the moment of loop creation 132rename() responsible for that would be holding filesystem lock and new parent 133would have to be equal to or a descendent of source. But that means that 134new parent had been equal to or a descendent of source since the moment when 135we had acquired filesystem lock and rename() would fail with -ELOOP in that 136case. 137 138While this locking scheme works for arbitrary DAGs, it relies on 139ability to check that directory is a descendent of another object. Current 140implementation assumes that directory graph is a tree. This assumption is 141also preserved by all operations (cross-directory rename on a tree that would 142not introduce a cycle will leave it a tree and link() fails for directories). 143 144Notice that "directory" in the above == "anything that might have 145children", so if we are going to introduce hybrid objects we will need 146either to make sure that link(2) doesn't work for them or to make changes 147in is_subdir() that would make it work even in presence of such beasts. 148