Re: [RFC] Improved inode number allocation for HTree

Andreas Dilger
2003-03-11T18:11:04Z

On Mar 11, 2003 14:41 +0100, Daniel Phillips wrote:
> On Tue 11 Mar 03 14:00, Helge Hafting wrote:
> > I'm happy nobody _can_ lock a directory like that. Think of it - unable
> > to create or delete files while some slow-moving program is traversing
> > the directory? Ouch. Plenty of options for DOS attacks too.
> > And how to do "rm *.bak" if rm locks the dir for traversal?
>
> <wishful thinking>
> Now that you mention it, just locking out create and rename during directory
> traversal would eliminate the pain. Delete is easy enough to handle during
> traversal. For a B-Tree, coalescing could simply be deferred until the
> traversal is finished, so reading the directory in physical storage order
> would be fine. Way, way cleaner than what we have to do now.
> </wishful thinking>

The other problem, of course, is that this would essentially mean that a
user-space application has a lock that prevents other processes (even the
kernel) from doing anything. At best we have to worry about an application
going sour, and at worst it is a DOS hole as Helge says.

How about something like:

death:
for each directory
while readdir(directory, large_buffer)
if (directory)
run death(directory)
readdir(directory, small_buffer)

sleep forever

The first getdents walks recursively down to the leaves, and then does a
single getdents with enough space to hold a single dirent, locking the
directory, but then never progressing. It also does the single-dirent
getdents on the way up the tree. Now, everything is locked out of the
entire directory tree.

Cheers, Andreas
--
Andreas Dilger