linux-ext4 - Re: [PATCH v2 5/6] jbd2: fix a race when checking checkpoint buffer busy

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Date:   Tue, 6 Jun 2023 09:59:46 +0200
From:   Jan Kara <jack@...e.cz>
To:     Zhang Yi <yi.zhang@...weicloud.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz, yi.zhang@...wei.com,
        yukuai3@...wei.com, chengzhihao1@...wei.com
Subject: Re: [PATCH v2 5/6] jbd2: fix a race when checking checkpoint buffer
 busy

On Tue 06-06-23 14:14:46, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@...wei.com>
> 
> Before removing checkpoint buffer from the t_checkpoint_list, we have to
> check both BH_Dirty and BH_Lock bits together to distinguish buffers
> have not been or were being written back. But __cp_buffer_busy() checks
> them separately, it first check lock state and then check dirty, the
> window between these two checks could be raced by writing back
> procedure, which locks buffer and clears buffer dirty before I/O
> completes. So it cannot guarantee checkpointing buffers been written
> back to disk if some error happens later. Finally, it may clean
> checkpoint transactions and lead to inconsistent filesystem.
> 
> jbd2_journal_forget() and __journal_try_to_free_buffer() also have the
> same problem (journal_unmap_buffer() escape from this issue since it's
> running under the buffer lock), so fix them through introducing a new
> helper to try holding the buffer lock and remove really clean buffer.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217490
> Cc: stable@...r.kernel.org
> Suggested-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

Just a type correction below:

> @@ -615,6 +619,34 @@ int __jbd2_journal_remove_checkpoint(struct journal_head *jh)
>  	return 1;
>  }
>  
> +/*
> + * Check the checkpoint buffer and try to remove it from the checkpoint
> + * list if it's clean. Returns -EBUSY if it is not clean, returns 1 if
> + * it frees the transaction, 0 otherwise.
> + *
> + * This function is called with j_list_lock held.
> + */
> +int jbd2_journal_try_remove_checkpoint(struct journal_head *jh)
> +{
> +	struct buffer_head *bh = jh2bh(jh);
> +
> +	if (!trylock_buffer(bh))
> +		return -EBUSY;
> +	if (buffer_dirty(bh)) {
> +		unlock_buffer(bh);
> +		return -EBUSY;
> +	}
> +	unlock_buffer(bh);
> +
> +	/*
> +	 * Buffer is clean and the IO has finished (we hold the buffer
							^^^ held

> +	 * lock) so the checkpoint is done. We can safely remove the
> +	 * buffer from this transaction.
> +	 */
> +	JBUFFER_TRACE(jh, "remove from checkpoint list");
> +	return __jbd2_journal_remove_checkpoint(jh);
> +}

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR