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btrfs: fix harmless race getting delayed ref head count when running delayed refs

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When running delayed references we are reading the number of ready delayed
ref heads without taking any lock which can make KCSAN report a race since
we can have concurrent tasks updating that number, such as for example
when freeing a tree block which will end up decrementing that counter or
when adding a new delayed ref while COWing a tree block which will
increment that counter.

This is a harmless race since running one more or one less delayed ref
head doesn't result in any problem, in the critical section of a
transaction commit we always run any remaining delayed refs and at that
point no one can create more.

So fix this harmless race by annotating the read with data_race().

Reported-by: cen zhang <zzzccc427@gmail.com>
Link: https://lore.kernel.org/linux-btrfs/CAFRLqsUCLMz0hY-GaPj1Z=fhkgRHjxVXHZ8kz0PvkFN0b=8L2Q@mail.gmail.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
2025-05-12 12:31:24 +01:00
committed by David Sterba
parent 4469e95fe5
commit 7dbfa4266c
1 changed files with 6 additions and 1 deletions
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7
fs/btrfs/extent-tree.c
View File

@@ -2006,7 +2006,12 @@ static noinline int __btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
delayed_refs = &trans->transaction->delayed_refs;
if (min_bytes == 0) {
max_count = delayed_refs->num_heads_ready;
/*
* We may be subject to a harmless race if some task is
* concurrently adding or removing a delayed ref, so silence
* KCSAN and similar tools.
*/
max_count = data_race(delayed_refs->num_heads_ready);
min_bytes = U64_MAX;
}