Merge git://git.kernel.org/pub/scm/linux/kernel/git/steve/gfs2-2.6-nmw

                   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.

	  GFS is perfect consistency -- changes made to the filesystem on one

	  machine show up immediately on all other machines in the cluster.

	  To use the GFS2 filesystem, you will need to enable one or more of

	  the below locking modules. Documentation and utilities for GFS2 can

	  To use the GFS2 filesystem in a cluster, you will need to enable

	  the locking module below. Documentation and utilities for GFS2 can

	  be found here: http://sources.redhat.com/cluster

config GFS2_FS_LOCKING_NOLOCK

	tristate "GFS2 \"nolock\" locking module"

	depends on GFS2_FS

	help

	  Single node locking module for GFS2.

	  Use this module if you want to use GFS2 on a single node without

	  its clustering features. You can still take advantage of the

	  large file support, and upgrade to running a full cluster later on

	  if required.

	  If you will only be using GFS2 in cluster mode, you do not need this

	  module.

	  The "nolock" lock module is now built in to GFS2 by default.

config GFS2_FS_LOCKING_DLM

	tristate "GFS2 DLM locking module"

	ops_fstype.o ops_inode.o ops_super.o quota.o \

	recovery.o rgrp.o super.o sys.o trans.o util.o

obj-$(CONFIG_GFS2_FS_LOCKING_NOLOCK) += locking/nolock/

obj-$(CONFIG_GFS2_FS_LOCKING_DLM) += locking/dlm/

	CREATE = 1,

};

enum {

	NO_WAIT = 0,

	WAIT = 1,

};

enum {

	NO_FORCE = 0,

	FORCE = 1,