linux-kernel - Re: [xfs-masters] Re: [BUG] Lockdep warning with XFS on 2.6.22-rc6

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Date:	Wed, 27 Jun 2007 16:42:08 +1000
From:	Tim Shimmin <tes@....com>
To:	xfs-masters@....sgi.com
CC:	Jarek Poplawski <jarkao2@...pl>, David Chinner <dgc@....com>,
	Ingo Molnar <mingo@...e.hu>,
	Satyam Sharma <satyam.sharma@...il.com>,
	Johannes Weiner <hannes-kernel@...urebad.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [xfs-masters] Re: [BUG] Lockdep warning with XFS on 2.6.22-rc6

Patch looks good, Dave.
(though, I stuffed up reviewing that bit of code previously:-)

Oh, previous typo: s/inodes at the some time/inodes at the same time/

--Tim

David Chinner wrote:
> On Tue, Jun 26, 2007 at 11:35:20AM +0200, Jarek Poplawski wrote:
>> On 26-06-2007 04:16, David Chinner wrote:
>>> It does both - parent-first/child-second and ascending inode # order,
>>> which is where the problem is. standing alone, these seem fine, but
>>> they don't appear to work when the child has a lower inode number
>>> than the parent.
>> ...
>>
>> >From xfs_inode.h:
>>
>> /*
>>  * Flags for lockdep annotations.
>>  *
>>  * XFS_I[O]LOCK_PARENT - for operations that require locking two inodes
>>  * (ie directory operations that require locking a directory inode and
>>  * an entry inode).  The first inode gets locked with this flag so it
>>  * gets a lockdep subclass of 1 and the second lock will have a lockdep
>>  * subclass of 0.
>>  *
>>  * XFS_I[O]LOCK_INUMORDER - for locking several inodes at the some time
>>  * with xfs_lock_inodes().  This flag is used as the starting subclass
>>  * and each subsequent lock acquired will increment the subclass by one.
>>  * So the first lock acquired will have a lockdep subclass of 2, the
>>  * second lock will have a lockdep subclass of 3, and so on.
>>  */
>>
>> I don't know xfs code, and probably miss something, but it seems
>> there could be some inconsistency: lockdep warning shows mr_lock/1
>> taken both before and after mr_lock (i.e. /0). According to the
>> above comment there should be always 1 before 0...
> 
> That just fired some rusty neurons.
> 
> #define XFS_IOLOCK_SHIFT        16
> #define XFS_IOLOCK_PARENT       (1 << XFS_IOLOCK_SHIFT)
> #define XFS_IOLOCK_INUMORDER    (2 << XFS_IOLOCK_SHIFT)
> 
> #define XFS_ILOCK_SHIFT         24
> #define XFS_ILOCK_PARENT        (1 << XFS_ILOCK_SHIFT)
> #define XFS_ILOCK_INUMORDER     (2 << XFS_ILOCK_SHIFT)
> 
> So, in a lock_mode parameter, the upper 8 bits are for the ILOCK lockdep
> subclass, and the 16..23 bits are for the IOLOCK lockdep subclass.
> 
> Where do we add them?
> 
> static inline int
> xfs_lock_inumorder(int lock_mode, int subclass)
> {
>         if (lock_mode & (XFS_IOLOCK_SHARED|XFS_IOLOCK_EXCL))
>                 lock_mode |= (subclass + XFS_IOLOCK_INUMORDER) << XFS_IOLOCK_SHIFT;
>         if (lock_mode & (XFS_ILOCK_SHARED|XFS_ILOCK_EXCL))
>                 lock_mode |= (subclass + XFS_ILOCK_INUMORDER) << XFS_ILOCK_SHIFT;
> 
>         return lock_mode;
> }
> 
> 
> OH, look at those nice overflow bugs in that in that code. We shift
> the XFS_IOLOCK_INUMORDER and XFS_ILOCK_INUMORDER bits out the far
> side of the lock_mode variable result in lock subclasses of 0-3 instead
> of 2-5....
> 
> Bugger, eh?
> 
> Patch below should fix this (untested).
> 
> Jarek - thanks for pointing what I should have seen earlier.
> 
> Cheers,
> 
> Dave.

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