Re: shrink_dcache_parent contention can make it stall for hours

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

On Thu, Oct 02, 2025 at 12:54:38PM +0100, Miroslav Crnić wrote:

> Option 1:
> Don't differentiate between work stealing and own work.
> Collect items for work stealing in the disposal list along with regular items.
> Fight out the contention per item with try_lock and continue through the list.

Currently disposal list is modified only by the thread that has created it;
which lock (if any) do you have in mind for protecting those?

> Option 2:
> Change shrink_dcache_parent to collect a dispose list for one unique parent.
> Split out unlinking from parent from __dentry_kill.
> Run over the disposal list in 2 passes.
> First pass does everything except unlink parent.
> Second pass does all unlinking under a single parent->d_lock grab.

Do you mean dentry_unlist()?  The problem is, how do you prevent those parents
looking inexplicably busy?  Cross-fs disposal list existing in parallel with fs
shutdown is very much possible; shrink_dcache_for_umount() should quietly steal
everything relevant from that and move on, leaving the empty husks on the
list to by freed by list owner once it gets to them.