Re: [PATCH] fs/dcache.c: re-add cond_resched() in shrink_dcache_parent()

[Al Viro]

On Sun, Apr 15, 2018 at 01:51:07AM +0100, Al Viro wrote:
> On Sat, Apr 14, 2018 at 02:47:21PM -0700, Linus Torvalds wrote:
> > On Sat, Apr 14, 2018 at 1:58 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > That breaks d_invalidate(), unfortunately.  Look at the termination
> > > conditions in the loop there...
> > 
> > Ugh. I was going to say "but that doesn't even use select_collect()",
> > but yeah, detach_and_collect() calls it.
> > 
> > It would be easy enough to just change the
> > 
> >                 if (!list_empty(&data.select.dispose))
> > 
> > there to
> > 
> >                 if (!list_empty(&data.select.found))
> > 
> > too.
> 
> You would have to do the same in check_and_drop() as well,
> and that brings back d_invalidate()/d_invalidate() livelock
> we used to have.  See 81be24d263db...
> 
> I'm trying to put something together, but the damn thing is
> full of potential livelocks, unfortunately ;-/  Will send
> a followup once I have something resembling a sane solution...

I really wonder if we should just do the following in
d_invalidate():
	* grab ->d_lock on victim, check if it's unhashed,
unlock and bugger off if it is.  Otherwise, unhash and unlock.
>From that point on any d_set_mounted() in the subtree will
fail.
	* shrink_dcache_parent() to reduce the subtree size.
	* go through the (hopefully shrunk) subtree, picking
mountpoints.  detach_mounts() for each of them.
	* shrink_dcache_parent() if any points had been
encountered, to kick the now-unpinned stuff.

As a side benefit, we could probably be gentler on rename_lock
in d_set_mounted() after that change...