tmpfs questions

[Jack Humphries]

Hi everyone,

I'm trying to study the FreeBSD tmpfs implementation as a personal
project, and I had a couple questions. I've been looking through the
code for a week and modifying various parts. I appreciate any help!

1. It seems that vnodes are locked before being passed to the various
VOP functions in tmpfs (because there is a call to
MPASS(VOP_ISLOCKED(vp)) near the beginning of each function).
Therefore, is the implicit assumption that a thread that holds the
vnode lock has exclusive access to the corresponding tmpfs_node
struct? In other words, is this why there are accesses to the tmpfs
node variables even though the tmpfs node is not locked? Note: I see
tn_interlock, but based on a comment above it in the source, it only
protects tn_vpstate and tn_status.

2. What is the duplicate node list for (tn_dupindex)? If I had to
guess, it seems to have something to do with the case where one thread
calls readdir on a directory while another is modifying the directory,
but I'm not sure. Can someone explain this deeper?

Thanks.

Jack Humphries