Re: [PATCH v2 1/4] dcache: sweep cached negative dentries to the end of list of siblings

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

On Tue, Feb 15, 2022 at 06:24:53PM -0800, Stephen Brennan wrote:

> It seems to me that, if we had taken a reference on child by
> incrementing the reference count prior to unlocking it, then
> dentry_unlist could never have been called, since we would never have
> made it into __dentry_kill. child would still be on the list, and any
> cursor (or sweep_negative) list updates would now be reflected in
> child->d_child.next. But dput is definitely not safe while holding a
> lock on a parent dentry (even more so now thanks to my patch), so that
> is out of the question.
> 
> Would dput_to_list be an appropriate solution to that issue? We can
> maintain a dispose list in d_walk and then for any dput which really
> drops the refcount to 0, we can handle them after d_walk is done. It
> shouldn't be that many dentries anyway.

	Interesting idea, but... what happens to behaviour of e.g.
shrink_dcache_parent()?  You'd obviously need to modify the test in
select_collect(), but then the selected dentries become likely candidates
for d_walk() itself wanting to move them over to its internal shrink list.
OTOH, __dput_to_list() will just decrement the count and skip the sucker
if it's already on a shrink list...

	It might work, but it really needs a careful analysis wrt.
parallel d_walk().  What happens when you have two threads hitting
shrink_dcache_parent() on two different places, one being an ancestor
of another?  That can happen in parallel, and currently it does work
correctly, but that's fairly delicate and there are places where a minor
change could turn O(n) into O(n^2), etc.

	Let me think about that - I'm not saying it's hopeless, and it
would be nice to avoid that subtlety in dentry_unlist(), but there
might be dragons.