Re: [PATCH] xfs/log: protect the logging content under xc_ctx_lock

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Oct 30, 2019 at 09:37:11PM +0800, Pingfan Liu wrote:
> xc_cil_lock is not enough to protect the integrity of a trans logging.
> Taking the scenario:
>   cpuA                                 cpuB                          cpuC
> 
>   xlog_cil_insert_format_items()
> 
>   spin_lock(&cil->xc_cil_lock)
>   link transA's items to xc_cil,
>      including item1
>   spin_unlock(&cil->xc_cil_lock)
>                                                                       xlog_cil_push() fetches transA's item under xc_cil_lock
>                                        issue transB, modify item1
>                                                                       xlog_write(), but now, item1 contains content from transB and we have a broken transA
> 
> Survive this race issue by putting under the protection of xc_ctx_lock.
> Meanwhile the xc_cil_lock can be dropped as xc_ctx_lock does it against
> xlog_cil_insert_items()

How did you trigger this race?  Is there a test case to reproduce, or
did you figure this out via code inspection?

--D

> 
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
> Cc: Brian Foster <bfoster@redhat.com>
> To: linux-xfs@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> ---
>  fs/xfs/xfs_log_cil.c | 35 +++++++++++++++++++----------------
>  1 file changed, 19 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 004af09..f8df3b5 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -723,22 +723,6 @@ xlog_cil_push(
>  	 */
>  	lv = NULL;
>  	num_iovecs = 0;
> -	spin_lock(&cil->xc_cil_lock);
> -	while (!list_empty(&cil->xc_cil)) {
> -		struct xfs_log_item	*item;
> -
> -		item = list_first_entry(&cil->xc_cil,
> -					struct xfs_log_item, li_cil);
> -		list_del_init(&item->li_cil);
> -		if (!ctx->lv_chain)
> -			ctx->lv_chain = item->li_lv;
> -		else
> -			lv->lv_next = item->li_lv;
> -		lv = item->li_lv;
> -		item->li_lv = NULL;
> -		num_iovecs += lv->lv_niovecs;
> -	}
> -	spin_unlock(&cil->xc_cil_lock);
>  
>  	/*
>  	 * initialise the new context and attach it to the CIL. Then attach
> @@ -783,6 +767,25 @@ xlog_cil_push(
>  	up_write(&cil->xc_ctx_lock);
>  
>  	/*
> +	 * cil->xc_cil_lock around this loop can be dropped, since xc_ctx_lock
> +	 * protects us against xlog_cil_insert_items().
> +	 */
> +	while (!list_empty(&cil->xc_cil)) {
> +		struct xfs_log_item	*item;
> +
> +		item = list_first_entry(&cil->xc_cil,
> +					struct xfs_log_item, li_cil);
> +		list_del_init(&item->li_cil);
> +		if (!ctx->lv_chain)
> +			ctx->lv_chain = item->li_lv;
> +		else
> +			lv->lv_next = item->li_lv;
> +		lv = item->li_lv;
> +		item->li_lv = NULL;
> +		num_iovecs += lv->lv_niovecs;
> +	}
> +
> +	/*
>  	 * Build a checkpoint transaction header and write it to the log to
>  	 * begin the transaction. We need to account for the space used by the
>  	 * transaction header here as it is not accounted for in xlog_write().
> -- 
> 2.7.5
>