'td' vs sys/fs/nfsserver/

[Konstantin Belousov]

On Tue, Mar 05, 2019 at 09:26:43AM +0000, Edward Napierala wrote:
> Hello.  As many of you know, right now, throughout the kernel,
> there’s the ‘td’ argument being passed over and over, even though
> we have a cheap way of accessing it by using ‘curthread’.  That’s
> suboptimal for obvious reasons.  It can’t be fixed ‘just like that’,
> as it would introduce a lot of code churn, and also possible bugs
> in case we miss a case where the ‘td’ is not equal to curthread.
> So, here's the big picture: this is what I'm _not_ intending
> to do at this point.
> 
> What I do intend doing is to go and try to fix it in a single kernel
> component, the NFS server.  The idea: drop the ‘td’ argument (in
> case of NFS server it’s usually spelled ‘p’ due to historical
> reasons) where it’s unused, which turns out to be quite often, and
> otherwise push ‘td’ down, function by function, so that when you
> review it it’s obvious that it’s equal to curthread.  There are
> three reasons to do this: first, it makes it very obvious that
> the
td passed to various VOPs it calls is indeed always curthread,
> and makes it easier to do the change described in the previous
> paragraph, should someone try it.  Second… well, it’s a cleanup:
> the NFS code passes it everywhere, and in many cases it's not used
> at all.  Third, and this is the man reason: it’s a good way to test
> the idea of pushing td down layer by layer without touching any
> APIs that are not local to the NFS server, to see if it works, and
> if it doesn't annoy people to much.  


So, since I’ve been asked to
> discuss it in public first: what do you guys think?

Lets put aside the importance of the problem.

I find the selection of the component to remove the 'td' argument somewhat
strange.  It is on top of the VFS stack, so any VOPs or other VFS functions
calls taking 'td' would require insertion of many curthread()s.  Why not
start from the bottom ?

I believe that looking at all in-tree filesystems and deciding which VOPs
and VFS_XXX methods do not need td because they do not use it, would give
more useful approach.  After td is elminated from VOPs which do not need
it, it should be rather uncontended to eleminate td from corresponding
callers of them.