Re: About the try to remove cross-release feature entirely by Ingo

[Byungchul Park]

On 1/3/2018 11:58 AM, Dave Chinner wrote:
> On Wed, Jan 03, 2018 at 11:28:44AM +0900, Byungchul Park wrote:
> > On 1/1/2018 7:18 PM, Matthew Wilcox wrote:
> > > On Sat, Dec 30, 2017 at 06:00:57PM -0500, Theodore Ts'o wrote:
> > > > Also, what to do with TCP connections which are created in userspace
> > > > (with some authentication exchanges happening in userspace), and then
> > > > passed into kernel space for use in kernel space, is an interesting
> > > > question.
> > >
> > > Yes!  I'd love to have a lockdep expert weigh in here.  I believe it's
> > > legitimate to change a lock's class after it's been used, essentially
> > > destroying it and reinitialising it.  If not, it should be because it's
> > > a reasonable design for an object to need different lock classes for
> > > different phases of its existance.
> >
> > I also think it should be done ultimately. And I think it's very much
> > hard since it requires to change the dependency graph of lockdep but
> > anyway possible. It's up to lockdep maintainer's will though..
>
> We used to do this in XFS to work around the fact that the memory
> reclaim context "locks" were too stupid to understand that an object
> referenced and locked above memory allocation could not be
> accessed below in memory reclaim because memory reclaim only accesses
> /unreferenced objects/. We played whack-a-mole with lockdep for
> years to get most of the false positives sorted out.
>
> Hence for a long time we had to re-initialise the lock context for
> the XFS inode iolock in ->evict_inode() so we could lock it for
> reclaim processing.  Eventually we ended up completely reworking the
> inode reclaim locking in XFS primarily to get rid of all the nasty
> lockdep hacks we had strewn throughout the code. It was ~2012 we
> got rid of the last inode re-init code, IIRC. Yeah:
>
> commit 4f59af758f9092bc7b266ca919ce6067170e5172
> Author: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Date:   Wed Jul 4 11:13:33 2012 -0400
>
>      xfs: remove iolock lock classes
>      
>      Now that we never take the iolock during inode reclaim we don't need
>      to play games with lock classes.
>      
>      Signed-off-by: Christoph Hellwig <hch@xxxxxx>
>      Reviewed-by: Rich Johnston <rjohnston@xxxxxxx>
>      Signed-off-by: Ben Myers <bpm@xxxxxxx>
>
> We still have problems with lockdep false positives w.r.t. memory
> allocation contexts, mainly with code that can be called from
> both above and below memory allocation contexts. We've finally
> got __GFP_NOLOCKDEP to be able to annotate memory allocation points
> within such code paths, but that doesn't help with locks....
>
> Byungchul, lockdep has a long, long history of having sharp edges
> and being very unfriendly to developers. We've all been scarred by
> lockdep at one time or another and so there's a fair bit of
> resistance to repeating past mistakes and allowing lockdep to
> inflict more scars on us....

As I understand what you suffered from.. I don't really want to
force it forward strongly.

So far, all problems have been handled by myself including the
final one e.i. the completion in submit_bio_wait() with the
invalidation if it's allowed. But yes, who knows the future? In
the future, that terrible thing you mentioned might or might
not happen because of cross-release.

I just felt like someone was misunderstanding what the problem
came from, what the problem was, how we could avoid it, why
cross-release should be removed and so on..

I believe the 3 ways I suggested can help, but I don't want to
strongly insist if all of you don't think so.

Thanks a lot anyway for your opinion.

--
Thanks,
Byungchul