Re: locking from VOP_INACTIVE to VOP_RECLAIM

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

> Date: Sun, 2 Apr 2017 16:34:20 +0200
> From: "J. Hannken-Illjes" <hannken%eis.cs.tu-bs.de@localhost>
> 
> Looks like your proposal needs some clarification regarding "vnode lock"
> and "the lock".
> 
> We have two vnode related locks:
> 
> - the "vnode lock", located as field "vi_lock" in "struct vnode_impl"
>   used by genfs_*lock() aka VOP_*LOCK.
> 
> - the "genfs lock" located as field "g_lock" in "struct genfs_node"
>   used by genfs_node_*lock().
> 
> Which lock do you want to be held across VOP_INACTIVE and destroyed
> from VOP_RECLAIM?

Aha!  That would explain why I had trouble tracking down why an assert
I was planning to add kept firing.  I was thinking that the genfs node
lock and the vnode lock were one and the same.

My proposal was about the vnode lock, not the genfs lock.  Since
VOP_RECLAIM does *not* destroy the vnode lock, and must leave it
either locked or unlocked, I suggest leaving it locked -- it's always
easier to think about subroutines that preserve the state of a lock
(locked to locked, or unlocked to unlocked) rather than changing it
(locked to unlocked or unlocked to locked).

(As an aside, we may have some bugs in ffs concerning use of the
struct inode::i_size field -- some callers seem to think access is
serialized by the genfs lock, while others, e.g. ufs_readwrite.c, seem
to use the vnode lock.  This needs to be documented more clearly...)