9f1585cb03
This patch adds a file describing the internals of GFS2's glock abstraction. Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
114 lines
5.5 KiB
Text
114 lines
5.5 KiB
Text
Glock internal locking rules
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------------------------------
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This documents the basic principles of the glock state machine
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internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h)
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has two main (internal) locks:
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1. A spinlock (gl_spin) which protects the internal state such
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as gl_state, gl_target and the list of holders (gl_holders)
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2. A non-blocking bit lock, GLF_LOCK, which is used to prevent other
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threads from making calls to the DLM, etc. at the same time. If a
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thread takes this lock, it must then call run_queue (usually via the
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workqueue) when it releases it in order to ensure any pending tasks
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are completed.
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The gl_holders list contains all the queued lock requests (not
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just the holders) associated with the glock. If there are any
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held locks, then they will be contiguous entries at the head
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of the list. Locks are granted in strictly the order that they
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are queued, except for those marked LM_FLAG_PRIORITY which are
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used only during recovery, and even then only for journal locks.
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There are three lock states that users of the glock layer can request,
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namely shared (SH), deferred (DF) and exclusive (EX). Those translate
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to the following DLM lock modes:
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Glock mode | DLM lock mode
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------------------------------
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UN | IV/NL Unlocked (no DLM lock associated with glock) or NL
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SH | PR (Protected read)
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DF | CW (Concurrent write)
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EX | EX (Exclusive)
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Thus DF is basically a shared mode which is incompatible with the "normal"
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shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O
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operations. The glocks are basically a lock plus some routines which deal
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with cache management. The following rules apply for the cache:
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Glock mode | Cache data | Cache Metadata | Dirty Data | Dirty Metadata
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--------------------------------------------------------------------------
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UN | No | No | No | No
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SH | Yes | Yes | No | No
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DF | No | Yes | No | No
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EX | Yes | Yes | Yes | Yes
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These rules are implemented using the various glock operations which
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are defined for each type of glock. Not all types of glocks use
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all the modes. Only inode glocks use the DF mode for example.
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Table of glock operations and per type constants:
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Field | Purpose
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----------------------------------------------------------------------------
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go_xmote_th | Called before remote state change (e.g. to sync dirty data)
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go_xmote_bh | Called after remote state change (e.g. to refill cache)
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go_inval | Called if remote state change requires invalidating the cache
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go_demote_ok | Returns boolean value of whether its ok to demote a glock
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| (e.g. checks timeout, and that there is no cached data)
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go_lock | Called for the first local holder of a lock
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go_unlock | Called on the final local unlock of a lock
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go_dump | Called to print content of object for debugfs file, or on
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| error to dump glock to the log.
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go_type; | The type of the glock, LM_TYPE_.....
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go_min_hold_time | The minimum hold time
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The minimum hold time for each lock is the time after a remote lock
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grant for which we ignore remote demote requests. This is in order to
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prevent a situation where locks are being bounced around the cluster
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from node to node with none of the nodes making any progress. This
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tends to show up most with shared mmaped files which are being written
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to by multiple nodes. By delaying the demotion in response to a
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remote callback, that gives the userspace program time to make
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some progress before the pages are unmapped.
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There is a plan to try and remove the go_lock and go_unlock callbacks
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if possible, in order to try and speed up the fast path though the locking.
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Also, eventually we hope to make the glock "EX" mode locally shared
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such that any local locking will be done with the i_mutex as required
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rather than via the glock.
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Locking rules for glock operations:
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Operation | GLF_LOCK bit lock held | gl_spin spinlock held
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-----------------------------------------------------------------
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go_xmote_th | Yes | No
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go_xmote_bh | Yes | No
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go_inval | Yes | No
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go_demote_ok | Sometimes | Yes
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go_lock | Yes | No
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go_unlock | Yes | No
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go_dump | Sometimes | Yes
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N.B. Operations must not drop either the bit lock or the spinlock
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if its held on entry. go_dump and do_demote_ok must never block.
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Note that go_dump will only be called if the glock's state
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indicates that it is caching uptodate data.
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Glock locking order within GFS2:
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1. i_mutex (if required)
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2. Rename glock (for rename only)
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3. Inode glock(s)
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(Parents before children, inodes at "same level" with same parent in
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lock number order)
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4. Rgrp glock(s) (for (de)allocation operations)
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5. Transaction glock (via gfs2_trans_begin) for non-read operations
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6. Page lock (always last, very important!)
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There are two glocks per inode. One deals with access to the inode
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itself (locking order as above), and the other, known as the iopen
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glock is used in conjunction with the i_nlink field in the inode to
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determine the lifetime of the inode in question. Locking of inodes
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is on a per-inode basis. Locking of rgrps is on a per rgrp basis.
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