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Message-ID: <01c15818-5765-408b-aff0-6c68b8c2a874@alu.unizg.hr>
Date:   Thu, 21 Sep 2023 22:15:49 +0200
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To:     linux-btrfs@vger.kernel.org
Cc:     Nikolay Borisov <nborisov@suse.com>, Chris Mason <clm@fb.com>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, linux-kernel@vger.kernel.org
From:   Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Subject: BUG: KCSAN: data-race in btrfs_sync_log [btrfs] / btrfs_update_inode
 [btrfs]
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Hi,

On the vanilla 6.6-rc2 torvalds kernel KCSAN reported this data-race:

[ 2690.990793] ==================================================================
[ 2690.991470] BUG: KCSAN: data-race in btrfs_sync_log [btrfs] / btrfs_update_inode [btrfs]

[ 2690.992804] write to 0xffff88811b57faf8 of 4 bytes by task 40555 on cpu 20:
[ 2690.992815] btrfs_sync_log (/home/marvin/linux/kernel/torvalds2/fs/btrfs/tree-log.c:2964) btrfs
[ 2690.993484] btrfs_sync_file (/home/marvin/linux/kernel/torvalds2/fs/btrfs/file.c:1954) btrfs
[ 2690.994149] vfs_fsync_range (/home/marvin/linux/kernel/torvalds2/fs/sync.c:188)
[ 2690.994161] __x64_sys_fsync (/home/marvin/linux/kernel/torvalds2/./include/linux/file.h:45 /home/marvin/linux/kernel/torvalds2/fs/sync.c:213 /home/marvin/linux/kernel/torvalds2/fs/sync.c:220 /home/marvin/linux/kernel/torvalds2/fs/sync.c:218 /home/marvin/linux/kernel/torvalds2/fs/sync.c:218)
[ 2690.994172] do_syscall_64 (/home/marvin/linux/kernel/torvalds2/arch/x86/entry/common.c:50 /home/marvin/linux/kernel/torvalds2/arch/x86/entry/common.c:80)
[ 2690.994186] entry_SYSCALL_64_after_hwframe (/home/marvin/linux/kernel/torvalds2/arch/x86/entry/entry_64.S:120)

[ 2690.994203] read to 0xffff88811b57faf8 of 4 bytes by task 5338 on cpu 21:
[ 2690.994214] btrfs_update_inode (/home/marvin/linux/kernel/torvalds2/fs/btrfs/transaction.h:175 /home/marvin/linux/kernel/torvalds2/fs/btrfs/inode.c:4016) btrfs
[ 2690.994877] btrfs_finish_one_ordered (/home/marvin/linux/kernel/torvalds2/fs/btrfs/inode.c:4028 /home/marvin/linux/kernel/torvalds2/fs/btrfs/inode.c:3139) btrfs
[ 2690.995541] btrfs_finish_ordered_io (/home/marvin/linux/kernel/torvalds2/fs/btrfs/inode.c:3230) btrfs
[ 2690.996205] finish_ordered_fn (/home/marvin/linux/kernel/torvalds2/fs/btrfs/ordered-data.c:304) btrfs
[ 2690.996871] btrfs_work_helper (/home/marvin/linux/kernel/torvalds2/fs/btrfs/async-thread.c:314) btrfs
[ 2690.997539] process_one_work (/home/marvin/linux/kernel/torvalds2/kernel/workqueue.c:2630)
[ 2690.997551] worker_thread (/home/marvin/linux/kernel/torvalds2/kernel/workqueue.c:2697 /home/marvin/linux/kernel/torvalds2/kernel/workqueue.c:2784)
[ 2690.997562] kthread (/home/marvin/linux/kernel/torvalds2/kernel/kthread.c:388)
[ 2690.997571] ret_from_fork (/home/marvin/linux/kernel/torvalds2/arch/x86/kernel/process.c:147)
[ 2690.997583] ret_from_fork_asm (/home/marvin/linux/kernel/torvalds2/arch/x86/entry/entry_64.S:312)

[ 2690.997598] value changed: 0x00000004 -> 0x00000005

[ 2690.997613] Reported by Kernel Concurrency Sanitizer on:
[ 2690.997621] CPU: 21 PID: 5338 Comm: kworker/u65:7 Tainted: G             L     6.6.0-rc2-kcsan-00143-gb5cbe7c00aa0 #41
[ 2690.997633] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
[ 2690.997640] Workqueue: btrfs-endio-write btrfs_work_helper [btrfs]
[ 2690.998311] ==================================================================

fs/btrfs/tree-log.c
-------------------------------------
2948         /*
2949          * We _must_ update under the root->log_mutex in order to make sure we
2950          * have a consistent view of the log root we are trying to commit at
2951          * this moment.
2952          *
2953          * We _must_ copy this into a local copy, because we are not holding the
2954          * log_root_tree->log_mutex yet.  This is important because when we
2955          * commit the log_root_tree we must have a consistent view of the
2956          * log_root_tree when we update the super block to point at the
2957          * log_root_tree bytenr.  If we update the log_root_tree here we'll race
2958          * with the commit and possibly point at the new block which we may not
2959          * have written out.
2960          */
2961         btrfs_set_root_node(&log->root_item, log->node);
2962         memcpy(&new_root_item, &log->root_item, sizeof(new_root_item));
2963
2964 →       root->log_transid++;
2965         log->log_transid = root->log_transid;
2966         root->log_start_pid = 0;
2967         /*
2968          * IO has been started, blocks of the log tree have WRITTEN flag set
2969          * in their headers. new modifications of the log will be written to
2970          * new positions. so it's safe to allow log writers to go in.
2971          */
2972         mutex_unlock(&root->log_mutex);

fs/btrfs/transaction.h
----------------------------------
170 static inline void btrfs_set_inode_last_trans(struct btrfs_trans_handle *trans,
171                                               struct btrfs_inode *inode)
172 {
173         spin_lock(&inode->lock);
174         inode->last_trans = trans->transaction->transid;
175         inode->last_sub_trans = inode->root->log_transid;
176         inode->last_log_commit = inode->last_sub_trans - 1;
177         spin_unlock(&inode->lock);
178 }

I am not certain whether the reader and writer side contend for the same lock, but it
seems that on the safe side would be putting the reader into READ_ONCE() to get a consistent
value?:

A diff speaks more than a thousand words:

----------------------
diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
index 6b309f8a99a8..b8cf86ce4c9e 100644
--- a/fs/btrfs/transaction.h
+++ b/fs/btrfs/transaction.h
@@ -172,7 +172,7 @@ static inline void btrfs_set_inode_last_trans(struct btrfs_trans_handle *trans,
  {
         spin_lock(&inode->lock);
         inode->last_trans = trans->transaction->transid;
-       inode->last_sub_trans = inode->root->log_transid;
+       inode->last_sub_trans = READ_ONCE(inode->root->log_transid);
         inode->last_log_commit = inode->last_sub_trans - 1;
         spin_unlock(&inode->lock);
  }
--

Best regards,
Mirsad Todorovac