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docs: filesystems: convert gfs2-glocks.txt to ReST 

- Add a SPDX header;
- Adjust document and section titles;
- Some whitespace fixes and new line breaks;
- Add table markups;
- Use notes markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
This commit is contained in:
Mauro Carvalho Chehab 2020-04-15 16:32:31 +02:00 committed by Andreas Gruenbacher
parent 3d77e6a880
commit 3700bec332
3 changed files with 86 additions and 64 deletions
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147
Documentation/filesystems/gfs2-glocks.txt → Documentation/filesystems/gfs2-glocks.rst
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@ -1,5 +1,8 @@
Glock internal locking rules
------------------------------
.. SPDX-License-Identifier: GPL-2.0
============================
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)
@ -24,24 +27,28 @@ 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)
========== ====== =====================================================
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
========== ========== ============== ========== ==============
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
@ -49,21 +56,23 @@ 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
============= =============================================================
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
@ -82,21 +91,25 @@ rather than via the glock.
Locking rules for glock operations:
Operation | GLF_LOCK bit lock held | gl_lockref.lock 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
============= ====================== =============================
Operation GLF_LOCK bit lock held gl_lockref.lock 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.
.. Note::
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:
@ -104,7 +117,7 @@ Glock locking order within GFS2:
2. Rename glock (for rename only)
3. Inode glock(s)
(Parents before children, inodes at "same level" with same parent in
lock number order)
lock number order)
4. Rgrp glock(s) (for (de)allocation operations)
5. Transaction glock (via gfs2_trans_begin) for non-read operations
6. i_rw_mutex (if required)
@ -117,8 +130,8 @@ 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
------------------
Glock Statistics
----------------
The stats are divided into two sets: those relating to the
super block and those relating to an individual glock. The
@ -173,8 +186,8 @@ 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")
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
@ -195,10 +208,13 @@ 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
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
@ -206,14 +222,16 @@ 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)
========= ================================================================
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
@ -224,9 +242,12 @@ 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
====== =======================================
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)

1
Documentation/filesystems/index.rst
View File

@ -69,6 +69,7 @@ Documentation for filesystem implementations.
f2fs
gfs2
gfs2-uevents
gfs2-glocks
hfs
hfsplus
hpfs

2
MAINTAINERS
View File

@ -7179,7 +7179,7 @@ L: cluster-devel@redhat.com
S: Supported
W: http://sources.redhat.com/cluster/
T: git git://git.kernel.org/pub/scm/linux/kernel/git/gfs2/linux-gfs2.git
F: Documentation/filesystems/gfs2*.txt
F: Documentation/filesystems/gfs2*
F: fs/gfs2/
F: include/uapi/linux/gfs2_ondisk.h