Re: [PATCH] btrfs: Fix race in read_extent_buffer_pages()

[Qu Wenruo]

在 2024/3/16 11:44, Tavian Barnes 写道:
> To prevent concurrent reads for the same extent buffer,
> read_extent_buffer_pages() performs these checks:
>
>      /* (1) */
>      if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
>          return 0;
>
>      /* (2) */
>      if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
>          goto done;
>
> At this point, it seems safe to start the actual read operation. Once
> that completes, end_bbio_meta_read() does
>
>      /* (3) */
>      set_extent_buffer_uptodate(eb);
>
>      /* (4) */
>      clear_bit(EXTENT_BUFFER_READING, &eb->bflags);
>
> Normally, this is enough to ensure only one read happens, and all other
> callers wait for it to finish before returning.  Unfortunately, there is
> a racey interleaving:
>
>      Thread A | Thread B | Thread C
>      ---------+----------+---------
>         (1)   |          |
>               |    (1)   |
>         (2)   |          |
>         (3)   |          |
>         (4)   |          |
>               |    (2)   |
>               |          |    (1)
>
> When this happens, thread B kicks of an unnecessary read. Worse, thread
> C will see UPTODATE set and return immediately, while the read from
> thread B is still in progress.  This race could result in tree-checker
> errors like this as the extent buffer is concurrently modified:
>
>      BTRFS critical (device dm-0): corrupted node, root=256
>      block=8550954455682405139 owner mismatch, have 11858205567642294356
>      expect [256, 18446744073709551360]
>
> Fix it by testing UPTODATE again after setting the READING bit, and if
> it's been set, skip the unnecessary read.
>
> Fixes: d7172f52e993 ("btrfs: use per-buffer locking for extent_buffer reading")
> Link: https://lore.kernel.org/linux-btrfs/CAHk-=whNdMaN9ntZ47XRKP6DBes2E5w7fi-0U3H2+PS18p+Pzw@xxxxxxxxxxxxxx/
> Link: https://lore.kernel.org/linux-btrfs/f51a6d5d7432455a6a858d51b49ecac183e0bbc9.1706312914.git.wqu@xxxxxxxx/
> Link: https://lore.kernel.org/linux-btrfs/c7241ea4-fcc6-48d2-98c8-b5ea790d6c89@xxxxxxx/
> Signed-off-by: Tavian Barnes <tavianator@xxxxxxxxxxxxxx>
Reviewed-by: Qu Wenruo <wqu@xxxxxxxx>

Thanks,
Qu
> ---
>   fs/btrfs/extent_io.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 7441245b1ceb..61594eaf1f89 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4333,6 +4333,19 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num,
>   	if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
>   		goto done;
>   
> +	/*
> +	 * Between the initial test_bit(EXTENT_BUFFER_UPTODATE) and the above
> +	 * test_and_set_bit(EXTENT_BUFFER_READING), someone else could have
> +	 * started and finished reading the same eb.  In this case, UPTODATE
> +	 * will now be set, and we shouldn't read it in again.
> +	 */
> +	if (unlikely(test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))) {
> +		clear_bit(EXTENT_BUFFER_READING, &eb->bflags);
> +		smp_mb__after_atomic();
> +		wake_up_bit(&eb->bflags, EXTENT_BUFFER_READING);
> +		return 0;
> +	}
> +
>   	clear_bit(EXTENT_BUFFER_READ_ERR, &eb->bflags);
>   	eb->read_mirror = 0;
>   	check_buffer_tree_ref(eb);