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

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

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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