DragonFly kernel List (threaded) for 2006-01
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Re: nnull
On 2006-01-02, Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx> wrote:
> What a nice Christmas present! I was hoping someone would come along
> and rewrite nullfs!
It's good to hear that there is someone who thinks this day is still fine
as Christmas, as I have a nice pile of unanswered Xmas greetings :)
> There is one issue with regards to overlayed namecache entries.
> The issue occurs when you try to do a rename() or other namespace
> operation and you have multiple namecache records refering to the
> same physical namespace. Since rename() locks the namespace by locking
> the related namecache records, it becomes possible for a namespace
> operation in the nullfs layer to race a namespace operation in the
> underlying filesystem layer. The system probably wouldn't be too happy
> when this happens.
Hm, I played a little bit with this. Here is a little test:
$ { cat flipflop.py; echo EOF; }
import os, sys
def flipflop(a, b):
while True:
try:
os.rename(a, b)
print os.path.basename(os.getcwd()), a, b
except OSError: pass
a, b = sys.argv[3:]
for dir in sys.argv[1:3]:
for x,y in ((a,b), (b,a)):
if os.fork() == 0:
os.chdir(dir)
flipflop(x, y)
EOF
$ mkdir A B
$ > A/a
$ mount_null A B
$ python2.4 flipflop.py A B a b
As of now, the effect not fatal... rather funny: after a while the dir(s) end
up empty.
Csaba
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