tmpfs questions

Konstantin Belousov kostikbel at gmail.com
Sat Jul 14 07:48:43 UTC 2018


On Fri, Jul 13, 2018 at 05:42:59PM -0700, Jack Humphries wrote:
> 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.
tmpfs nodes are protected by the vnode locks.  Note that vnode lock
can be held exclusive or shared.  Typically, only exclusive lock allows
the code to modify the node, owning shared vnode lock only means that
the node can be read safely.

Interlock exists because node state must be examined sometimes without
owning the vnode lock, in non-sleepable context.

> 
> 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?
As an optimization, children of the directory node are organized into
red/black tree, which cannot hold two entries with the same key value.
If a second entry with the existing key is attempted to be created in
the directory, it is added to the dup list instead.


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