[PATCH 04/11] VFS: introduce dentry_lookup_continue()

[Al Viro]

On Tue, Aug 12, 2025 at 12:25:07PM +1000, NeilBrown wrote:
> A few callers operate on a dentry which they already have - unlike the
> normal case where a lookup proceeds an operation.
> 
> For these callers dentry_lookup_continue() is provided where other
> callers would use dentry_lookup().  The call will fail if, after the
> lock was gained, the child is no longer a child of the given parent.
> 
> There are a couple of callers that want to lock a dentry in whatever
> its current parent is.  For these a NULL parent can be passed, in which
> case ->d_parent is used.  In this case the call cannot fail.
> 
> The idea behind the name is that the actual lookup occurred some time
> ago, and now we are continuing with an operation on the dentry.
> 
> When the operation completes done_dentry_lookup() must be called.  An
> extra reference is taken when the dentry_lookup_continue() call succeeds
> and will be dropped by done_dentry_lookup().
> 
> This will be used in smb/server, ecryptfs, and overlayfs, each of which
> have their own lock_parent() or parent_lock() or similar; and a few
> other places which lock the parent but don't check if the parent is
> still correct (often because rename isn't supported so parent cannot be
> incorrect).

I would really like the see the conversion of these callers.  You are
asking for a buy-in for a primitive with specific semantics; that's
hard to review without seeing how it will be used.