linux-kernel - [PATCH v2] btrfs: fix data race when accessing the inode's disk_i_size at btrfs_drop

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Message-Id: <20241202133515.92286-1-zhenghaoran154@gmail.com>
Date: Mon,  2 Dec 2024 21:35:15 +0800
From: Hao-ran Zheng <zhenghaoran154@...il.com>
To: clm@...com,
	josef@...icpanda.com,
	dsterba@...e.com,
	linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: baijiaju1990@...il.com,
	zhenghaoran154@...il.com,
	21371365@...a.edu.cn
Subject: [PATCH v2] btrfs: fix data race when accessing the inode's disk_i_size at btrfs_drop_extents()

A data race occurs when the function `insert_ordered_extent_file_extent()`
and the function `btrfs_inode_safe_disk_i_size_write()` are executed
concurrently. The function `insert_ordered_extent_file_extent()` is not
locked when reading inode->disk_i_size, causing
`btrfs_inode_safe_disk_i_size_write()` to cause data competition when
writing inode->disk_i_size, thus affecting the value of `modify_tree`.

Since the impact of data race is limited, it is recommended to use the
`data_race` mark judgment.

The specific call stack that appears during testing is as follows:
============DATA_RACE============
 btrfs_drop_extents+0x89a/0xa060 [btrfs]
 insert_reserved_file_extent+0xb54/0x2960 [btrfs]
 insert_ordered_extent_file_extent+0xff5/0x1760 [btrfs]
 btrfs_finish_one_ordered+0x1b85/0x36a0 [btrfs]
 btrfs_finish_ordered_io+0x37/0x60 [btrfs]
 finish_ordered_fn+0x3e/0x50 [btrfs]
 btrfs_work_helper+0x9c9/0x27a0 [btrfs]
 process_scheduled_works+0x716/0xf10
 worker_thread+0xb6a/0x1190
 kthread+0x292/0x330
 ret_from_fork+0x4d/0x80
 ret_from_fork_asm+0x1a/0x30
============OTHER_INFO============
 btrfs_inode_safe_disk_i_size_write+0x4ec/0x600 [btrfs]
 btrfs_finish_one_ordered+0x24c7/0x36a0 [btrfs]
 btrfs_finish_ordered_io+0x37/0x60 [btrfs]
 finish_ordered_fn+0x3e/0x50 [btrfs]
 btrfs_work_helper+0x9c9/0x27a0 [btrfs]
 process_scheduled_works+0x716/0xf10
 worker_thread+0xb6a/0x1190
 kthread+0x292/0x330
 ret_from_fork+0x4d/0x80
 ret_from_fork_asm+0x1a/0x30
=================================

Signed-off-by: Hao-ran Zheng <zhenghaoran154@...il.com>
---
V1->V2: Modify patch description and format
---
 fs/btrfs/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 4fb521d91b06..f9631713f67d 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -242,7 +242,7 @@ int btrfs_drop_extents(struct btrfs_trans_handle *trans,
 	if (args->drop_cache)
 		btrfs_drop_extent_map_range(inode, args->start, args->end - 1, false);
 
-	if (args->start >= inode->disk_i_size && !args->replace_extent)
+	if (data_race(args->start >= inode->disk_i_size && !args->replace_extent))
 		modify_tree = 0;
 
 	update_refs = (btrfs_root_id(root) != BTRFS_TREE_LOG_OBJECTID);
-- 
2.34.1