Re: [PATCH] locking/lockdep: Remove the cross-release locking checks

[Byungchul Park]

On Sat, Dec 16, 2017 at 6:15 AM, Theodore Ts'o <tytso@xxxxxxx> wrote:
> On Fri, Dec 15, 2017 at 05:39:25PM +0900, Byungchul Park wrote:
>>
>> All locks should belong to one class if each path of acquisition
>> can be switchable each other within the class at any time.
>> Otherwise, they should belong to a different class.
>
> OK, so let's go back to my case of a Network Block Device with a local
> file system mounted on it, which is then exported via NFS.
>
> So an incoming TCP packet can go into the NFS server subsystem, then
> be processed by local disk file system, which then does an I/O
> operation to the NBD device, which results in an TCP packet being sent
> out.  Then the response will come back over TCP, into the NBD block
> layer, then into the local disk file system, and that will result in
> an outgoing response to the TCP connection for the NFS protocol.
>
> In order to avoid cross release problems, all locks associated with
> the incoming TCP connection will need to be classified as belonging to
> a different class as the outgoing TCP connection.  Correct?  One
> solution might be to put every single TCP connection into a separate
> class --- but that will explode the number of lock classes which
> Lockdep will need to track, and there is a limited number of lock
> classes (set at compile time) that Lockdep can track.  So if that
> doesn't work, we will have to put something ugly which manually makes
> certain TCP connections "magic" and require them to be put into a
> separate class than all other TCP connections, which will get
> collapsed into a single class.  Basically, any TCP connection which is
> either originated by the kernel, or passed in from userspace into the
> kernel and used by some kernel subsystem, will have to be assigned its
> own lockdep class.
>
> If the TCP connection gets closed, we don't need to track that lockdep
> class any more.  (Or if a device mapper device is torn down, we
> similarly don't need any unique lockdep classes created for that
> device mapper device.)  Is there a way to tell lockdep that a set of
> lockdep classes can be released so we can recover the kernel memory to
> be used to track some new TCP connection or some new device mapper
> device?

Right. I also think lockdep should be able to reflect that
kind of dynamic situations to do a better job.

The fact that kernel works well w/o that work doesn't
mean current status is perfect, in my opinion.

As you know, lockdep is running within very limited
environment so it's very hard to achieve it.

However, anyway, I think that's a problem and should
be solved by modifying lockdep core. Actually, that had
been one on my to-dos, if allowed.

For some waiters, for which this is only solution to play
with cross-release, I think we can invalidate those
waiters for now, while all others still get benefit.

We have added acquire annotations manually to
consider waiters one by one, and I am sure it's going
to continue in the future.

IMO, considering all waiters at once and fixing false
positives in a right way or invalidating one by one is
better than considering waiters one by one as is, of
course, while keeping off by default.

-- 
Thanks,
Byungchul