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btrfs: add comment about locking to btrfs_split_ordered_extent()

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There are subtle details about why the root's ordered_extent_lock is held,
so add a comment mentioning them.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Filipe Manana 2024-06-03 16:50:31 +01:00 committed by David Sterba
parent ac1f580c10
commit c18ca3c960
1 changed files with 17 additions and 0 deletions
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17
fs/btrfs/ordered-data.c
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@ -1247,6 +1247,23 @@ struct btrfs_ordered_extent *btrfs_split_ordered_extent(
/* One ref for the tree. */
refcount_inc(&new->refs);
/*
* Take the root's ordered_extent_lock to avoid a race with
* btrfs_wait_ordered_extents() when updating the disk_bytenr and
* disk_num_bytes fields of the ordered extent below. And we disable
* IRQs because the inode's ordered_tree_lock is used in IRQ context
* elsewhere.
*
* There's no concern about a previous caller of
* btrfs_wait_ordered_extents() getting the trimmed ordered extent
* before we insert the new one, because even if it gets the ordered
* extent before it's trimmed and the new one inserted, right before it
* uses it or during its use, the ordered extent might have been
* trimmed in the meanwhile, and it missed the new ordered extent.
* There's no way around this and it's harmless for current use cases,
* so we take the root's ordered_extent_lock to fix that race during
* trimming and silence tools like KCSAN.
*/
spin_lock_irq(&root->ordered_extent_lock);
spin_lock(&inode->ordered_tree_lock);
/* Remove from tree once */