linux-kernel - Re: [PATCH] vfs: check dentry is still valid in get

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Date:   Tue, 18 Jan 2022 05:58:14 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Dave Chinner <david@...morbit.com>
Cc:     Brian Foster <bfoster@...hat.com>, Ian Kent <raven@...maw.net>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Miklos Szeredi <miklos@...redi.hu>,
        David Howells <dhowells@...hat.com>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH] vfs: check dentry is still valid in get_link()

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()?