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Date: Tue, 19 Mar 2024 06:32:12 +1030
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Subject: Re: [PATCH 2/2] btrfs: WARN if EXTENT_BUFFER_UPTODATE is set while
 reading
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To: Tavian Barnes <tavianator@tavianator.com>, linux-btrfs@vger.kernel.org
Cc: Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
 David Sterba <dsterba@suse.com>, linux-kernel@vger.kernel.org
References: <cover.1710769876.git.tavianator@tavianator.com>
 <d4a055317bdb8ecbd7e6d9bdb5ebb074fa93f7f8.1710769876.git.tavianator@tavianator.com>
From: Qu Wenruo <wqu@suse.com>
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在 2024/3/19 00:26, Tavian Barnes 写道:
> We recently tracked down a race condition that triggered a read for an
> extent buffer with EXTENT_BUFFER_UPTODATE already set.  While this read
> was in progress, other concurrent readers would see the UPTODATE bit and
> return early as if the read was already complete, making accesses to the
> extent buffer conflict with the read operation that was overwriting it.
> 
> Add a WARN_ON() to end_bbio_meta_read() for this situation to make
> similar races easier to spot in the future.
> 
> Signed-off-by: Tavian Barnes <tavianator@tavianator.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu
> ---
>   fs/btrfs/extent_io.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 46173dcfde4f..0024ea20bfc4 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4285,6 +4285,13 @@ static void end_bbio_meta_read(struct btrfs_bio *bbio)
>   	struct folio_iter fi;
>   	u32 bio_offset = 0;
>   
> +	/*
> +	 * If the extent buffer is marked UPTODATE before the read operation
> +	 * completes, other calls to read_extent_buffer_pages() will return
> +	 * early without waiting for the read to finish, causing data races.
> +	 */
> +	WARN_ON(test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags));
> +
>   	eb->read_mirror = bbio->mirror_num;
>   
>   	if (uptodate &&