On Thu, May 31, 2012 at 1:01 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>
> Right. By default it's 90 seconds before we'll give up on the client.
So a slightly buggy client can basically DoS the server by getting a
delegation and then crashing or something. Everybody else that tries
to read that directory (not that file) will be dead in the water.
Definitely not good.
> I hate that too, and originally tried to avoid it with something like:
>
> retry:
> acquire locks
> lookup inode
> ret = try_to_break_deleg(inode);
> if (ret)
> drop locks
> really_break_deleg(inode);
> goto retry;
> ... do the real work ...
> drop locks
>
> I felt like I was making already complicated code logic like rename's
> even harder to follow.
I do think it's the only thing we can reasonably do. I'd love to have
some kind of per-dentry lock for unlink/rename, but we don't.
Long-term, we really do need to do something about the directory
locking, though, because it's also a huge problem for readdir()
concurrency. Or at least it used to be (samba in particular). Making
it an rwsem might help readdir a tiny amount, but I suspect people
actually depend on the mutex in readdir right now.
> And those operations don't really know the inode till they acquire the
> locks, so in pathological cases that could continue forever.
I suspect at some point you just have to say "screw it".
Linus
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