Re: [PATCH] btrfs: update device path inode time instead of bd_inode

[Christoph Hellwig <hch@lst.de>]

On Thu, Oct 14, 2021 at 12:00:10PM -0400, Josef Bacik wrote:
> 1 fs/btrfs/volumes.c  update_dev_time      1900 generic_update_time(d_inode(path.dentry), &now, S_MTIME | S_CTIME);

This is the onde we're talking about.

> 4 fs/inode.c          inode_update_time    1789 return generic_update_time(inode, time, flags);

This is update_time().

And all others are ->update_time instances.

> > Looking at this a bit more I think the right fix is to simply revert the
> > offending commit.  The lockdep complains was due to changes issues in the
> > loop driver and has been fixed in the loop driver in the meantime.
> > 
> 
> Where were they fixed?  And it doesn't fix the fact that we're calling open on a
> device, so any change at all to the loop device is going to end us back up in
> this spot because we end up with the ->lo_mutex in our dependency chain.  I want
> to avoid this by not calling open, and that means looking up the inode and doing
> operations without needing to go through the full file open path.

This all looks like the open_mutex (formerly bd_mutex) vs lo_mutex inside
and outside chains, and they were fixed.

> The best thing for btrfs here is to export the update_time() helper and call
> that to avoid all the baggage that comes from opening the block device.  Thanks,

update_time is a bit too low-level for an export as it requires a fair
effort to call it the right way.