Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

[Byungchul Park]

On Fri, Mar 03, 2017 at 08:50:03AM +0900, Byungchul Park wrote:
> On Thu, Mar 02, 2017 at 06:39:49AM -0800, Matthew Wilcox wrote:
> > On Thu, Mar 02, 2017 at 01:45:35PM +0900, byungchul.park wrote:
> > > From: Matthew Wilcox [mailto:willy@xxxxxxxxxxxxx]
> > > > On Tue, Feb 28, 2017 at 07:15:47PM +0100, Peter Zijlstra wrote:
> > > > > (And we should not be returning to userspace with locks held anyway --
> > > > > lockdep already has a check for that).
> > > > 
> > > > Don't we return to userspace with page locks held, eg during async
> > > > directio?
> > > 
> > > Hello,
> > > 
> > > I think that the check when returning to user with crosslocks held
> > > should be an exception. Don't you think so?
> > 
> > Oh yes.  We have to keep the pages locked during reads, and we have to
> > return to userspace before I/O is complete, therefore we have to return
> > to userspace with pages locked.  They'll be unlocked by the interrupt
> > handler in page_endio().
> 
> Agree.
> 
> > Speaking of which ... this feature is far too heavy for use in production
> > on pages.  You're almost trebling the size of struct page.  Can we
> > do something like make all struct pages share the same lockdep_map?
> > We'd have to not complain about holding one crossdep lock and acquiring
> > another one of the same type, but with millions of pages in the system,
> > it must surely be creating a gargantuan graph right now?
> 
> Um.. I will try it for page locks to work with one lockmap. That is also
> what Peterz pointed out and what I worried about when implementing..

I've thought it more and it seems not to be good. We could not use
subclass feature if we make page locks work with only one lockmap
instance. And there are several things we have to give up, that are,
things using each field in struct lockdep_map. So now, I'm not sure I
should change the current implementation. What do you think about it?