Re: [PATCH] xfs_file_last_byte() needs to acquire ilock

Felix Blyakher
2009-04-24T21:42:46Z

On Apr 23, 2009, at 10:46 PM, Lachlan McIlroy wrote:

>
> ----- "Eric Sandeen" <sandeen@sandeen.net> wrote:
>
>> Lachlan McIlroy wrote:
>>> We had some systems crash with this stack:
>>>
>>> [<a00000010000cb20>] ia64_leave_kernel+0x0/0x280
>>> [<a00000021291ca00>] xfs_bmbt_get_startoff+0x0/0x20 [xfs]
>>> [<a0000002129080b0>] xfs_bmap_last_offset+0x210/0x280 [xfs]
>>> [<a00000021295b010>] xfs_file_last_byte+0x70/0x1a0 [xfs]
>>> [<a00000021295b200>] xfs_itruncate_start+0xc0/0x1a0 [xfs]
>>> [<a0000002129935f0>] xfs_inactive_free_eofblocks+0x290/0x460 [xfs]
>>> [<a000000212998fb0>] xfs_release+0x1b0/0x240 [xfs]
>>> [<a0000002129ad930>] xfs_file_release+0x70/0xa0 [xfs]
>>> [<a000000100162ea0>] __fput+0x1a0/0x420
>>> [<a000000100163160>] fput+0x40/0x60
>>>
>>> The problem here is that xfs_file_last_byte() does not acquire the
>>> inode lock and can therefore race with another thread that is
>> modifying
>>> the extext list. While xfs_bmap_last_offset() is trying to lookup
>>> what was the last extent some extents were merged and the extent
>> list
>>> shrunk so the index we lookup is now beyond the end of the extent
>> list
>>> and potentially in a freed buffer.
>>>
>>> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
>>> index e7ae08d..cf62d9d 100644
>>> --- a/fs/xfs/xfs_inode.c
>>> +++ b/fs/xfs/xfs_inode.c
>>> @@ -1258,8 +1258,10 @@ xfs_file_last_byte(
>>
>> /*
>> * Only check for blocks beyond the EOF if the extents have
>> * been read in. This eliminates the need for the inode
>> lock,
>> * and it also saves us from looking when it really isn't
>>> * necessary.
>>> */
>>
>> I suppose that comment should be modified too, and maybe the commit
>> log
>> should say why, exactly, it was wrong? :)
> Ha, I didn't even read the comment! It's still kind of correct in
> that we wont have to get the inode lock if the extents have not been
>
> read in.

I'd still think the comments could be made less confusing
if we're adding the inode lock here.

Felix

>
>
>>
>> -Eric
>>
>>> if (ip->i_df.if_flags & XFS_IFEXTENTS) {
>>> + xfs_ilock(ip, XFS_ILOCK_SHARED);
>>> error = xfs_bmap_last_offset(NULL, ip, &last_block,
>>> XFS_DATA_FORK);
>>> + xfs_iunlock(ip, XFS_ILOCK_SHARED);
>>> if (error) {
>>> last_block = 0;
>>> }
>>>
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>>>
>>
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