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

Nikolay Borisov
2017-07-18T18:27:46Z

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

>
> 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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