linux-kernel - Re: nfsd changes for 3.5

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Date:	Thu, 31 May 2012 13:17:26 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: nfsd changes for 3.5

On Thu, May 31, 2012 at 1:01 PM, J. Bruce Fields <bfields@...ldses.org> 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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