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Subject[PATCH 11/18] [GFS2] Glock documentation
Date
From: Steven Whitehouse <swhiteho@redhat.com>

This patch adds a file describing the internals of GFS2's glock
abstraction.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

diff --git a/Documentation/filesystems/gfs2-glocks.txt b/Documentation/filesystems/gfs2-glocks.txt
new file mode 100644
index 0000000..4dae9a3
--- /dev/null
+++ b/Documentation/filesystems/gfs2-glocks.txt
@@ -0,0 +1,114 @@
+ Glock internal locking rules
+ ------------------------------
+
+This documents the basic principles of the glock state machine
+internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h)
+has two main (internal) locks:
+
+ 1. A spinlock (gl_spin) which protects the internal state such
+ as gl_state, gl_target and the list of holders (gl_holders)
+ 2. A non-blocking bit lock, GLF_LOCK, which is used to prevent other
+ threads from making calls to the DLM, etc. at the same time. If a
+ thread takes this lock, it must then call run_queue (usually via the
+ workqueue) when it releases it in order to ensure any pending tasks
+ are completed.
+
+The gl_holders list contains all the queued lock requests (not
+just the holders) associated with the glock. If there are any
+held locks, then they will be contiguous entries at the head
+of the list. Locks are granted in strictly the order that they
+are queued, except for those marked LM_FLAG_PRIORITY which are
+used only during recovery, and even then only for journal locks.
+
+There are three lock states that users of the glock layer can request,
+namely shared (SH), deferred (DF) and exclusive (EX). Those translate
+to the following DLM lock modes:
+
+Glock mode | DLM lock mode
+------------------------------
+ UN | IV/NL Unlocked (no DLM lock associated with glock) or NL
+ SH | PR (Protected read)
+ DF | CW (Concurrent write)
+ EX | EX (Exclusive)
+
+Thus DF is basically a shared mode which is incompatible with the "normal"
+shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O
+operations. The glocks are basically a lock plus some routines which deal
+with cache management. The following rules apply for the cache:
+
+Glock mode | Cache data | Cache Metadata | Dirty Data | Dirty Metadata
+--------------------------------------------------------------------------
+ UN | No | No | No | No
+ SH | Yes | Yes | No | No
+ DF | No | Yes | No | No
+ EX | Yes | Yes | Yes | Yes
+
+These rules are implemented using the various glock operations which
+are defined for each type of glock. Not all types of glocks use
+all the modes. Only inode glocks use the DF mode for example.
+
+Table of glock operations and per type constants:
+
+Field | Purpose
+----------------------------------------------------------------------------
+go_xmote_th | Called before remote state change (e.g. to sync dirty data)
+go_xmote_bh | Called after remote state change (e.g. to refill cache)
+go_inval | Called if remote state change requires invalidating the cache
+go_demote_ok | Returns boolean value of whether its ok to demote a glock
+ | (e.g. checks timeout, and that there is no cached data)
+go_lock | Called for the first local holder of a lock
+go_unlock | Called on the final local unlock of a lock
+go_dump | Called to print content of object for debugfs file, or on
+ | error to dump glock to the log.
+go_type; | The type of the glock, LM_TYPE_.....
+go_min_hold_time | The minimum hold time
+
+The minimum hold time for each lock is the time after a remote lock
+grant for which we ignore remote demote requests. This is in order to
+prevent a situation where locks are being bounced around the cluster
+from node to node with none of the nodes making any progress. This
+tends to show up most with shared mmaped files which are being written
+to by multiple nodes. By delaying the demotion in response to a
+remote callback, that gives the userspace program time to make
+some progress before the pages are unmapped.
+
+There is a plan to try and remove the go_lock and go_unlock callbacks
+if possible, in order to try and speed up the fast path though the locking.
+Also, eventually we hope to make the glock "EX" mode locally shared
+such that any local locking will be done with the i_mutex as required
+rather than via the glock.
+
+Locking rules for glock operations:
+
+Operation | GLF_LOCK bit lock held | gl_spin spinlock held
+-----------------------------------------------------------------
+go_xmote_th | Yes | No
+go_xmote_bh | Yes | No
+go_inval | Yes | No
+go_demote_ok | Sometimes | Yes
+go_lock | Yes | No
+go_unlock | Yes | No
+go_dump | Sometimes | Yes
+
+N.B. Operations must not drop either the bit lock or the spinlock
+if its held on entry. go_dump and do_demote_ok must never block.
+Note that go_dump will only be called if the glock's state
+indicates that it is caching uptodate data.
+
+Glock locking order within GFS2:
+
+ 1. i_mutex (if required)
+ 2. Rename glock (for rename only)
+ 3. Inode glock(s)
+ (Parents before children, inodes at "same level" with same parent in
+ lock number order)
+ 4. Rgrp glock(s) (for (de)allocation operations)
+ 5. Transaction glock (via gfs2_trans_begin) for non-read operations
+ 6. Page lock (always last, very important!)
+
+There are two glocks per inode. One deals with access to the inode
+itself (locking order as above), and the other, known as the iopen
+glock is used in conjunction with the i_nlink field in the inode to
+determine the lifetime of the inode in question. Locking of inodes
+is on a per-inode basis. Locking of rgrps is on a per rgrp basis.
+
--
1.5.1.2


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