On Nov 13, 2008, at 8:37 AM, Dan Cross wrote:
On Wed, Nov 12, 2008 at 2:13 PM, <[EMAIL PROTECTED]> wrote:
stat(5) specifies exclusive-access files, which we do use for
locking.
In what sense is that not `doing locking'? It's not POSIX byte-range
read- or write-locking per fcntl, but it's not clear to me how often
that's actually useful. In quite a few situations, having a single
process directly access a resource and forcing all other accesses
through that process suffices.
For special purposes, the canonical solution is probably just to have
a ctl file that accepts commands to manipulate locks. It's a bit of a
protocol dance in the sense of the calling program having some smarts
about what order it does things in, but wouldn't require changes to 9P
proper.
I recall when we were talking about adding a Tflush message around the
time of 9P2000 and Rob or Russ or someone saying, "that's really the
job of a fileserver." I didn't fully appreciate what they meant at
that time, thinking solely in terms of disk-based "file servers"
(instead of the more abstract notion of a fileserver as a collection
of related resources with well-defined operations on them) like kenfs,
but it dawned on me (much) later that they were just referring to some
sort of ctl file and a command along the lines of "sync <fid>".
Locking is a natural abstraction of that kind of thing; it could
easily be abstracted into a library function that does the right
thing, much as dial() does.
But along the very same line of thought -- wouldn't it also then be
much more reasonable to stick with an "alternative aname"
approach when adopting 9P for symlinks, FIFOs and the
rest of the POSIX paraphernalia?
You pay with a slightly more complicated server, but you
reap a huge benefit in a form of:
term% srvssh unix unix-fs
term% mount /srv/unix-fs /n/unix-fs
term% mount /srv/unix-fs /n/unix-fs-symlinks posix-symlinks
# Look ma, I'm creating a symbolic link from a Plan9 terminal
# I'll name it after you and it'll point to the root
term% echo / > /n/unix-fs-symlinks/home/glenda/mama
# And I can even do readlink()
term% cat /n/unix-fs-symlinks/home/glenda/mama
/
# Best of all -- it all works as expected
term% cd /n/unix-fs/home/glenda/mama
term% ls
bin cdrom etc initrd initrd.img.old lib32 lost+found
mnt proc sbin sys usr vmlinuz
boot dev home initrd.img lib lib64 media
opt root srv tmp var vmlinuz.old
Thanks,
Roman.
