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

Darrick J. Wong
2017-07-18T18:33:59Z

On Tue, Jul 18, 2017 at 09:27:43PM +0300, Nikolay Borisov wrote:
>
>
> On 18.07.2017 21:15, Darrick J. Wong wrote:
> > 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. :)
>
> https://www.spinics.net/lists/linux-xfs/msg05918.html

Ah, ok. That's what I suspected, so I'm glad to have confirmation.

--D

>
>
> >
> > 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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