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_callback	 | Called if the DLM sends a callback to drop this lock
  go_flags	 | GLOF_ASPACE is set, if the glock has an address space
                   | associated with it
  
  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
  go_callback   |       Sometimes (N/A)   |       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.
  In general we prefer to lock local locks prior to cluster locks.
  
                              Glock Statistics
                             ------------------
  
  The stats are divided into two sets: those relating to the
  super block and those relating to an individual glock. The
  super block stats are done on a per cpu basis in order to
  try and reduce the overhead of gathering them. They are also
  further divided by glock type. All timings are in nanoseconds.
  
  In the case of both the super block and glock statistics,
  the same information is gathered in each case. The super
  block timing statistics are used to provide default values for
  the glock timing statistics, so that newly created glocks
  should have, as far as possible, a sensible starting point.
  The per-glock counters are initialised to zero when the
  glock is created. The per-glock statistics are lost when
  the glock is ejected from memory.
  
  The statistics are divided into three pairs of mean and
  variance, plus two counters. The mean/variance pairs are
  smoothed exponential estimates and the algorithm used is
  one which will be very familiar to those used to calculation
  of round trip times in network code. See "TCP/IP Illustrated,
  Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement",
  p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards.
  Unlike the TCP/IP Illustrated case, the mean and variance are
  not scaled, but are in units of integer nanoseconds.
  
  The three pairs of mean/variance measure the following
  things:
  
   1. DLM lock time (non-blocking requests)
   2. DLM lock time (blocking requests)
   3. Inter-request time (again to the DLM)
  
  A non-blocking request is one which will complete right
  away, whatever the state of the DLM lock in question. That
  currently means any requests when (a) the current state of
  the lock is exclusive, i.e. a lock demotion (b) the requested
  state is either null or unlocked (again, a demotion) or (c) the
  "try lock" flag is set. A blocking request covers all the other
  lock requests.
  
  There are two counters. The first is there primarily to show
  how many lock requests have been made, and thus how much data
  has gone into the mean/variance calculations. The other counter
  is counting queuing of holders at the top layer of the glock
  code. Hopefully that number will be a lot larger than the number
  of dlm lock requests issued.
  
  So why gather these statistics? There are several reasons
  we'd like to get a better idea of these timings:
  
  1. To be able to better set the glock "min hold time"
  2. To spot performance issues more easily
  3. To improve the algorithm for selecting resource groups for
  allocation (to base it on lock wait time, rather than blindly
  using a "try lock")
  
  Due to the smoothing action of the updates, a step change in
  some input quantity being sampled will only fully be taken
  into account after 8 samples (or 4 for the variance) and this
  needs to be carefully considered when interpreting the
  results.
  
  Knowing both the time it takes a lock request to complete and
  the average time between lock requests for a glock means we
  can compute the total percentage of the time for which the
  node is able to use a glock vs. time that the rest of the
  cluster has its share. That will be very useful when setting
  the lock min hold time.
  
  Great care has been taken to ensure that we
  measure exactly the quantities that we want, as accurately
  as possible. There are always inaccuracies in any
  measuring system, but I hope this is as accurate as we
  can reasonably make it.
  
  Per sb stats can be found here:
  /sys/kernel/debug/gfs2/<fsname>/sbstats
  Per glock stats can be found here:
  /sys/kernel/debug/gfs2/<fsname>/glstats
  
  Assuming that debugfs is mounted on /sys/kernel/debug and also
  that <fsname> is replaced with the name of the gfs2 filesystem
  in question.
  
  The abbreviations used in the output as are follows:
  
  srtt     - Smoothed round trip time for non-blocking dlm requests
  srttvar  - Variance estimate for srtt
  srttb    - Smoothed round trip time for (potentially) blocking dlm requests
  srttvarb - Variance estimate for srttb
  sirt     - Smoothed inter-request time (for dlm requests)
  sirtvar  - Variance estimate for sirt
  dlm      - Number of dlm requests made (dcnt in glstats file)
  queue    - Number of glock requests queued (qcnt in glstats file)
  
  The sbstats file contains a set of these stats for each glock type (so 8 lines
  for each type) and for each cpu (one column per cpu). The glstats file contains
  a set of these stats for each glock in a similar format to the glocks file, but
  using the format mean/variance for each of the timing stats.
  
  The gfs2_glock_lock_time tracepoint prints out the current values of the stats
  for the glock in question, along with some addition information on each dlm
  reply that is received:
  
  status - The status of the dlm request
  flags  - The dlm request flags
  tdiff  - The time taken by this specific request
  (remaining fields as per above list)