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

Darrick J. Wong
2017-07-18T18:15:42Z

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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