quoted

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Looking at it again, it seems that sock_gettstamp() should
actually deal with this gracefully: it will return a -EINVAL
error condition if the timestamp remains at the
SK_DEFAULT_STAMP initial value, which is probably
just as appropriate (or better) as the current -ENOTTY
default, and if we are actually recording timestamps, we
might just as well report them.

Yes, that's a nice solution. There is always some risk in changing
error codes. But ioctl callers should be able to support newly
implemented functionality. Even if partially implemented and
returning ENOENT instead of ENOIOCTLCMD.

Ok, so do you think we should stay with the current version
for now, and change the two points later, or should I rework
it to integrate the locking and removing the callback?

I suppose the series actually gets nicer without the
callback, since I can simply add the generic timestamping
implementation first, and then remove the dead ioctl
handlers.

Agreed. I would add the locks in a separate patch, if only on the
off-chance that lockdep discovers something and it will be easier
to bisect and revise independently. I can also follow up with that
patch outside this set, of course.