linux-kernel - Re: [2.6.27-rc4] XFS i_lock vs i

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Date:	Tue, 26 Aug 2008 22:34:40 +0100
From:	"Daniel J Blueman" <daniel.blueman@...il.com>
To:	"Dave Chinner" <david@...morbit.com>
Cc:	"Christoph Hellwig" <hch@....de>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Lachlan McIlroy" <lachlan@....com>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>, xfs@....sgi.com
Subject: Re: [2.6.27-rc4] XFS i_lock vs i_iolock...

On Tue, Aug 26, 2008 at 9:13 PM, Daniel J Blueman
<daniel.blueman@...il.com> wrote:
> Hi Dave,
>
> On Tue, Aug 26, 2008 at 3:45 AM, Dave Chinner <david@...morbit.com> wrote:
>> On Mon, Aug 25, 2008 at 11:55:32PM +0200, Christoph Hellwig wrote:
>>> On Mon, Aug 25, 2008 at 08:59:33AM +0200, Peter Zijlstra wrote:
>>> > How can you take two locks in one go? It seems to me you always need to
>>> > take them one after another, and as soon as you do that, you have
>>> > ordering constraints.
>>>
>>> Yes, you would.  Except that in all other places we only have a single
>>> iolock involved, so the ordering of the second iolock and second ilock
>>> don't matter.
>>>
>>> Because of that I think declaring that xfs_lock_two_inodes can just
>>> lock on lock type at a time might be the better solution.
>>
>> Agreed. Patch below.
>>
>> Cheers,
>>
>> Dave.
>> --
>> Dave Chinner
>> david@...morbit.com
>>
>> XFS: prevent lockdep false positives when locking two inodes
>>
>> If we call xfs_lock_two_inodes() to grab both the iolock and
>> the ilock, then drop the ilocks on both inodes, then grab
>> them again (as xfs_swap_extents() does) then lockdep will
>> report a locking order problem. This is a false positive.
>>
>> To avoid this, disallow xfs_lock_two_inodes() fom locking both
>> inode locks at once - force calers to make two separate calls.
>> This means that nested dropping and regaining of the ilocks
>> will retain the same lockdep subclass and so lockdep will
>> not see anything wrong with this code.
>>
>> Signed-off-by: Dave Chinner <david@...morbit.com>
>> ---
>>  fs/xfs/xfs_dfrag.c    |    9 ++++++++-
>>  fs/xfs/xfs_vnodeops.c |   10 ++++++++++
>>  2 files changed, 18 insertions(+), 1 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_dfrag.c b/fs/xfs/xfs_dfrag.c
>> index 760f4c5..75b0cd4 100644
>> --- a/fs/xfs/xfs_dfrag.c
>> +++ b/fs/xfs/xfs_dfrag.c
>> @@ -149,7 +149,14 @@ xfs_swap_extents(
>>
>>        sbp = &sxp->sx_stat;
>>
>> -       xfs_lock_two_inodes(ip, tip, lock_flags);
>> +       /*
>> +        * we have to do two separate lock calls here to keep lockdep
>> +        * happy. If we try to get all the locks in one call, lock will
>> +        * report false positives when we drop the ILOCK and regain them
>> +        * below.
>> +        */
>> +       xfs_lock_two_inodes(ip, tip, XFS_IOLOCK_EXCL);
>> +       xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);
>>        locked = 1;
>>
>>        /* Verify that both files have the same format */
>> diff --git a/fs/xfs/xfs_vnodeops.c b/fs/xfs/xfs_vnodeops.c
>> index f108102..cb1b5fd 100644
>> --- a/fs/xfs/xfs_vnodeops.c
>> +++ b/fs/xfs/xfs_vnodeops.c
>> @@ -1836,6 +1836,12 @@ again:
>>  #endif
>>  }
>>
>> +/*
>> + * xfs_lock_two_inodes() can only be used to lock one type of lock
>> + * at a time - the iolock or the ilock, but not both at once. If
>> + * we lock both at once, lockdep will report false positives saying
>> + * we have violated locking orders.
>> + */
>>  void
>>  xfs_lock_two_inodes(
>>        xfs_inode_t             *ip0,
>> @@ -1846,7 +1852,11 @@ xfs_lock_two_inodes(
>>        int                     attempts = 0;
>>        xfs_log_item_t          *lp;
>>
>> +#ifdef DEBUG
>> +       if (lock_mode & (XFS_IOLOCK_SHARED|XFS_IOLOCK_EXCL))
>> +               ASSERT((lock_mode & (XFS_ILOCK_SHARED|XFS_ILOCK_EXCL)) == 0);
>>        ASSERT(ip0->i_ino != ip1->i_ino);
>> +#endif
>>
>>        if (ip0->i_ino > ip1->i_ino) {
>>                temp = ip0;
>
> Good to get your patch and HCH's ack...thanks!
>
> I'll pursue testing and touchdown in < 24 hrs.

Excellent - confirmed it addresses the lockdep report I was seeing
before and doesn't introduce any regressions.

Thanks,
  Daniel
-- 
Daniel J Blueman
--
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