Date: Mon, 4 Aug 2008 17:03:46 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Duane Griffin" <duaneg@dghda.com>
Cc: linux-kernel@vger.kernel.org, sct@redhat.com, linux-ext4@vger.kernel.org,
       Sami Liedes <sliedes@cc.hut.fi>
Subject: Re: [PATCH] jbd: abort instead of waiting for nonexistent
 transactions
Message-Id: <20080804170346.613238b8.akpm@linux-foundation.org>
In-Reply-To: <1217893895-29165-1-git-send-email-duaneg@dghda.com>
References: <1217893895-29165-1-git-send-email-duaneg@dghda.com>
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On Tue,  5 Aug 2008 00:51:34 +0100 "Duane Griffin" <duaneg@dghda.com> wrote:

> The __log_wait_for_space function sits in a loop checkpointing transactions
> until there is sufficient space free in the journal. However, if there are
> no transactions to be processed (e.g. because the free space calculation is
> wrong due to a corrupted filesystem) it will never progress.
> 
> Check for space being required when no transactions are outstanding and
> abort the journal instead of endlessly looping.
> 
> This patch fixes the bug reported by Sami Liedes at:
> http://bugzilla.kernel.org/show_bug.cgi?id=10976
> 
> Signed-off-by: Duane Griffin <duaneg@dghda.com>
> Tested-by: Sami Liedes <sliedes@cc.hut.fi>
> ---
> diff --git a/fs/jbd/checkpoint.c b/fs/jbd/checkpoint.c
> index a5432bb..9fac177 100644
> --- a/fs/jbd/checkpoint.c
> +++ b/fs/jbd/checkpoint.c
> @@ -126,14 +126,29 @@ void __log_wait_for_space(journal_t *journal)
>  
>  		/*
>  		 * Test again, another process may have checkpointed while we
> -		 * were waiting for the checkpoint lock
> +		 * were waiting for the checkpoint lock. If there are no
> +		 * outstanding transactions there is nothing to checkpoint and
> +		 * we can't make progress. Abort the journal in this case.
>  		 */
>  		spin_lock(&journal->j_state_lock);
> +		spin_lock(&journal->j_list_lock);
>  		nblocks = jbd_space_needed(journal);
>  		if (__log_space_left(journal) < nblocks) {
> +			int chkpt = journal->j_checkpoint_transactions != NULL;
> +
> +			spin_unlock(&journal->j_list_lock);
>  			spin_unlock(&journal->j_state_lock);
> -			log_do_checkpoint(journal);
> +			if (chkpt) {
> +				log_do_checkpoint(journal);
> +			} else {
> +				printk(KERN_ERR "%s: no transactions\n",
> +				       __func__);
> +				journal_abort(journal, 0);
> +			}
> +
>  			spin_lock(&journal->j_state_lock);
> +		} else {
> +			spin_unlock(&journal->j_list_lock);
>  		}
>  		mutex_unlock(&journal->j_checkpoint_mutex);
>  	}

I don't expect that the additional taking of j_list_lock in here does
anything useful.

Plus..  after j_list_lock has been dropped, new transactions could
theoretically appear at journal->j_checkpoint_transactions, so we
_could_ reclaim more journal space.  But a) that probably couldn't
happen due to ->j_state_lock and lots of other things and b) it's
hopelessly theoretical even if it _could_ happen, methinks.  Just
sayin'..

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