Re: [PATCH] vfs: check dentry is still valid in get_link()

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Jan 18, 2022 at 03:12:53PM +1100, Dave Chinner wrote:

> No, that just creates a black hole where the VFS inode has been
> destroyed but the XFS inode cache doesn't know it's been trashed.
> Hence setting XFS_IRECLAIMABLE needs to remain in the during
> ->destroy_inode, otherwise the ->lookup side of the cache will think
> that are currently still in use by the VFS and hand them straight
> back out without going through the inode recycling code.
> 
> i.e. XFS_IRECLAIMABLE is the flag that tells xfs_iget() that the VFS
> part of the inode has been torn down, and that it must go back
> through VFS re-initialisation before it can be re-instantiated as a
> VFS inode.

OK...

> It would also mean that the inode will need to go through two RCU
> grace periods before it gets reclaimed, because XFS uses RCU
> protected inode cache lookups internally (e.g. for clustering dirty
> inode writeback) and so freeing the inode from the internal
> XFS inode cache requires RCU freeing...

Wait a minute.  Where is that RCU delay of yours, relative to
xfs_vn_unlink() and xfs_vn_rename() (for target)?  And where does
it happen in case of e.g. open() + unlink() + close()?