linux-ext4 - [PATCH 5/9] ext4: Document lock ordering

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Date:	Wed,  4 Nov 2015 17:18:36 +0100
From:	Jan Kara <jack@...e.com>
To:	Ted Tso <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	dan.j.williams@...el.com, Jan Kara <jack@...e.com>
Subject: [PATCH 5/9] ext4: Document lock ordering

We have enough locks that it's probably worth documenting the lock
ordering rules we have in ext4.

Signed-off-by: Jan Kara <jack@...e.com>
---
 fs/ext4/super.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 61fad9e33bb9..152f719a0c62 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -85,6 +85,36 @@ static void ext4_unregister_li_request(struct super_block *sb);
 static void ext4_clear_request_list(void);
 static int ext4_reserve_clusters(struct ext4_sb_info *, ext4_fsblk_t);
 
+/*
+ * Lock ordering
+ *
+ * Note the difference between i_mmap_sem (EXT4_I(inode)->i_mmap_sem) and
+ * i_mmap_rwsem (inode->i_mmap_rwsem)!
+ *
+ * page fault path:
+ * mmap_sem -> sb_start_pagefault -> i_mmap_sem (r) -> transaction start ->
+ *   page lock -> i_data_sem (rw)
+ *
+ * buffered write path:
+ * sb_start_write -> i_mutex -> mmap_sem
+ * sb_start_write -> i_mutex -> transaction start -> page lock ->
+ *   i_data_sem (rw)
+ *
+ * truncate:
+ * sb_start_write -> i_mutex -> EXT4_STATE_DIOREAD_LOCK (w) -> i_mmap_sem (w) ->
+ *   i_mmap_rwsem (w) -> page lock
+ * sb_start_write -> i_mutex -> EXT4_STATE_DIOREAD_LOCK (w) -> i_mmap_sem (w) ->
+ *   transaction start -> i_data_sem (rw)
+ *
+ * direct IO:
+ * sb_start_write -> i_mutex -> EXT4_STATE_DIOREAD_LOCK (r) -> mmap_sem
+ * sb_start_write -> i_mutex -> EXT4_STATE_DIOREAD_LOCK (r) ->
+ *   transaction start -> i_data_sem (rw)
+ *
+ * writepages:
+ * transaction start -> page lock(s) -> i_data_sem (rw)
+ */
+
 #if !defined(CONFIG_EXT2_FS) && !defined(CONFIG_EXT2_FS_MODULE) && defined(CONFIG_EXT4_USE_FOR_EXT2)
 static struct file_system_type ext2_fs_type = {
 	.owner		= THIS_MODULE,
-- 
2.1.4

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