Re: [PATCH, RFC] xfs: fix multi-AG deadlock in xfs_bunmapi

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

On Tue, Jul 18, 2017 at 07:25:45PM +0200, Christoph Hellwig wrote:
> Just like in the allocator we must avoid touching multiple AGs out of
> order when freeing blocks, as freeing still locks the AGF and can cause
> the same AB-BA deadlocks as in the allocation path.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reported-by: Nikolay Borisov <n.borisov.lkml@gmail.com>

Looks ok, though I wonder what the bug report looked like. :)

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

> ---
>  fs/xfs/libxfs/xfs_bmap.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 0a9880777c9c..935adde72a8b 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -5435,6 +5435,7 @@ __xfs_bunmapi(
>  	xfs_fsblock_t		sum;
>  	xfs_filblks_t		len = *rlen;	/* length to unmap in file */
>  	xfs_fileoff_t		max_len;
> +	xfs_agnumber_t		prev_agno = NULLAGNUMBER, agno;
>  
>  	trace_xfs_bunmap(ip, bno, len, flags, _RET_IP_);
>  
> @@ -5534,6 +5535,17 @@ __xfs_bunmapi(
>  		 */
>  		del = got;
>  		wasdel = isnullstartblock(del.br_startblock);
> +
> +		/*
> +		 * Make sure we don't touch multiple AGF headers out of order
> +		 * in a single transaction, as that could cause AB-BA deadlocks.
> +		 */
> +		if (!wasdel) {
> +			agno = XFS_FSB_TO_AGNO(mp, del.br_startblock);
> +			if (prev_agno != NULLAGNUMBER && prev_agno > agno)
> +				break;
> +			prev_agno = agno;
> +		}
>  		if (got.br_startoff < start) {
>  			del.br_startoff = start;
>  			del.br_blockcount -= start - got.br_startoff;
> -- 
> 2.11.0
> 
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