Re: [Ocfs2-devel] [RFC] Should we revert commit "ocfs2: take inode lock in ocfs2_iop_set/get_acl()"? or other ideas?

Re: [Ocfs2-devel] [RFC] Should we revert commit "ocfs2: take inode lock in ocfs2_iop_set/get_acl()"? or other ideas?

[Christoph Hellwig]

Hi Eric,

I've added linux-fsdevel to the cc list as this should get a bit
broader attention.

On Wed, Oct 19, 2016 at 01:19:40PM +0800, Eric Ren wrote:
> Mostly, we can avoid recursive locking by writing code carefully. However, as
> the deadlock issues have proved out, it's very hard to handle the routines
> that are called directly by vfs. For instance:
> 
>     const struct inode_operations ocfs2_file_iops = {
>             .permission     = ocfs2_permission,
>             .get_acl        = ocfs2_iop_get_acl,
>             .set_acl        = ocfs2_iop_set_acl,
>     };
> 
> 
> ocfs2_permission() and ocfs2_iop_get/set_acl() both call ocfs2_inode_lock().
> The problem is that the call chain of ocfs2_permission() includes *_acl().

What do you actually protect in ocfs2_permission?  It's a trivial
wrapper around generic_permission which just looks at the VFS inode.

I think the right fix is to remove ocfs2_permission entirely and use
the default VFS implementation.  That both solves your locking problem,
and it will also get you RCU lookup instead of dropping out of
RCU mode all the time.

Re: [RFC] Should we revert commit "ocfs2: take inode lock in ocfs2_iop_set/get_acl()"? or other ideas?

[Eric Ren]

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Hi Christoph!

Thanks for your attention.

On 10/28/2016 02:20 PM, Christoph Hellwig wrote:
> Hi Eric,
>
> I've added linux-fsdevel to the cc list as this should get a bit
> broader attention.
>
> On Wed, Oct 19, 2016 at 01:19:40PM +0800, Eric Ren wrote:
>> Mostly, we can avoid recursive locking by writing code carefully. However, as
>> the deadlock issues have proved out, it's very hard to handle the routines
>> that are called directly by vfs. For instance:
>>
>>      const struct inode_operations ocfs2_file_iops = {
>>              .permission     = ocfs2_permission,
>>              .get_acl        = ocfs2_iop_get_acl,
>>              .set_acl        = ocfs2_iop_set_acl,
>>      };
>>
>>
>> ocfs2_permission() and ocfs2_iop_get/set_acl() both call ocfs2_inode_lock().
>> The problem is that the call chain of ocfs2_permission() includes *_acl().
> What do you actually protect in ocfs2_permission?  It's a trivial
> wrapper around generic_permission which just looks at the VFS inode.
Yes, it is.

https://github.com/torvalds/linux/blob/master/fs/ocfs2/file.c#L1321
---
ocfs2_permission
       ocfs2_inode_lock()
generic_permission
ocfs2_inode_unlock
>
> I think the right fix is to remove ocfs2_permission entirely and use
> the default VFS implementation.  That both solves your locking problem,
> and it will also get you RCU lookup instead of dropping out of
> RCU mode all the time.
But, from my understanding, the pair of ocfs2_inode_lock/unlock() is used to prevent any 
concurrent changes
to the permission of the inode on the other cluster node while we are checking on it. It's a 
common  case for cluster
filesystem, such as GFS2: https://github.com/torvalds/linux/blob/master/fs/gfs2/inode.c#L1777

Thanks for your suggestion again!
Eric
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