1                   Glock internal locking rules
2                  ------------------------------
3
4This documents the basic principles of the glock state machine
5internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h)
6has two main (internal) locks:
7
8 1. A spinlock (gl_spin) which protects the internal state such
9    as gl_state, gl_target and the list of holders (gl_holders)
10 2. A non-blocking bit lock, GLF_LOCK, which is used to prevent other
11    threads from making calls to the DLM, etc. at the same time. If a
12    thread takes this lock, it must then call run_queue (usually via the
13    workqueue) when it releases it in order to ensure any pending tasks
14    are completed.
15
16The gl_holders list contains all the queued lock requests (not
17just the holders) associated with the glock. If there are any
18held locks, then they will be contiguous entries at the head
19of the list. Locks are granted in strictly the order that they
20are queued, except for those marked LM_FLAG_PRIORITY which are
21used only during recovery, and even then only for journal locks.
22
23There are three lock states that users of the glock layer can request,
24namely shared (SH), deferred (DF) and exclusive (EX). Those translate
25to the following DLM lock modes:
26
27Glock mode    | DLM lock mode
28------------------------------
29    UN        |    IV/NL  Unlocked (no DLM lock associated with glock) or NL
30    SH        |    PR     (Protected read)
31    DF        |    CW     (Concurrent write)
32    EX        |    EX     (Exclusive)
33
34Thus DF is basically a shared mode which is incompatible with the "normal"
35shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O
36operations. The glocks are basically a lock plus some routines which deal
37with cache management. The following rules apply for the cache:
38
39Glock mode   |  Cache data | Cache Metadata | Dirty Data | Dirty Metadata
40--------------------------------------------------------------------------
41    UN       |     No      |       No       |     No     |      No
42    SH       |     Yes     |       Yes      |     No     |      No
43    DF       |     No      |       Yes      |     No     |      No
44    EX       |     Yes     |       Yes      |     Yes    |      Yes
45
46These rules are implemented using the various glock operations which
47are defined for each type of glock. Not all types of glocks use
48all the modes. Only inode glocks use the DF mode for example.
49
50Table of glock operations and per type constants:
51
52Field            | Purpose
53----------------------------------------------------------------------------
54go_xmote_th      | Called before remote state change (e.g. to sync dirty data)
55go_xmote_bh      | Called after remote state change (e.g. to refill cache)
56go_inval         | Called if remote state change requires invalidating the cache
57go_demote_ok     | Returns boolean value of whether its ok to demote a glock
58                 | (e.g. checks timeout, and that there is no cached data)
59go_lock          | Called for the first local holder of a lock
60go_unlock        | Called on the final local unlock of a lock
61go_dump          | Called to print content of object for debugfs file, or on
62                 | error to dump glock to the log.
63go_type          | The type of the glock, LM_TYPE_.....
64go_min_hold_time | The minimum hold time
65
66The minimum hold time for each lock is the time after a remote lock
67grant for which we ignore remote demote requests. This is in order to
68prevent a situation where locks are being bounced around the cluster
69from node to node with none of the nodes making any progress. This
70tends to show up most with shared mmaped files which are being written
71to by multiple nodes. By delaying the demotion in response to a
72remote callback, that gives the userspace program time to make
73some progress before the pages are unmapped.
74
75There is a plan to try and remove the go_lock and go_unlock callbacks
76if possible, in order to try and speed up the fast path though the locking.
77Also, eventually we hope to make the glock "EX" mode locally shared
78such that any local locking will be done with the i_mutex as required
79rather than via the glock.
80
81Locking rules for glock operations:
82
83Operation     |  GLF_LOCK bit lock held |  gl_spin spinlock held
84-----------------------------------------------------------------
85go_xmote_th   |       Yes               |       No
86go_xmote_bh   |       Yes               |       No
87go_inval      |       Yes               |       No
88go_demote_ok  |       Sometimes         |       Yes
89go_lock       |       Yes               |       No
90go_unlock     |       Yes               |       No
91go_dump       |       Sometimes         |       Yes
92
93N.B. Operations must not drop either the bit lock or the spinlock
94if its held on entry. go_dump and do_demote_ok must never block.
95Note that go_dump will only be called if the glock's state
96indicates that it is caching uptodate data.
97
98Glock locking order within GFS2:
99
100 1. i_mutex (if required)
101 2. Rename glock (for rename only)
102 3. Inode glock(s)
103    (Parents before children, inodes at "same level" with same parent in
104     lock number order)
105 4. Rgrp glock(s) (for (de)allocation operations)
106 5. Transaction glock (via gfs2_trans_begin) for non-read operations
107 6. Page lock  (always last, very important!)
108
109There are two glocks per inode. One deals with access to the inode
110itself (locking order as above), and the other, known as the iopen
111glock is used in conjunction with the i_nlink field in the inode to
112determine the lifetime of the inode in question. Locking of inodes
113is on a per-inode basis. Locking of rgrps is on a per rgrp basis.
114
115