Do the pidhashtbl_locks added by r340742 need to be sx locks?

[Rick Macklem]

Hi,

I finally got around to looking at what effect replacing pfind_locked() with
pfind() has for the NFSv4 client and it is broken.

The problem is that the NFS code needs to call some variant of "pfind()" while
holding a mutex lock. The current _pfind() code uses the pidhashtbl_locks,
which are "sx" locks.

There are a few ways to fix this:
1 - Create a custom version of _pfind() for the NFS client with the sx_X() calls
      removed, plus replace the locking of allproc_lock with locking of all the
      pidhashtbl_locks, so that the "sx" locks are acquired before the mutex.
      --> Not very efficient, but since it is only done once/sec, I can live with it.
2 - Similar to the above, but still lock the allproc_lock and use a loop of
     FOREACH_PROC_IN_SYSTEM(p) instead of a hash list for the pid in the
     custom pfind(). (I don't know if this would be preferable to locking all
     the pidhashtbl_locks for other users of pfind()?)
3 - Convert the pidhashtbl_locks to mutexes. Then the NFS client doesn't need
     to acquire any proc related locks and it just works.
     I can't see anywhere that "sleeps" while holding the pidhashtbl_locks, so I
     think they can be converted, although I haven't tried it yet?

>From my perspective, #3 seems the better solution.
What do others think?

rick