[OpenAFS] Re: Crazy DAFS problem (with log)

Andrew Deason adeason@sinenomine.net
Tue, 22 Mar 2011 15:13:01 -0500

On Mon, 21 Mar 2011 23:25:36 -0500
Andrew Deason <adeason@sinenomine.net> wrote:

> However, I don't think this fully explains the behavior. I haven't
> checked it out yet (and will not today), but this shouldn't really be
> causing the fd to be left open. We are calling IH_REALLYCLOSE when the
> volume goes offline, and we do appear to be going through the code
> path that closes all relevant file descriptors. But after that is
> done, I still see an FdHandle_t with a refcount of 0, in the _OPEN
> state holding open the problematic inode (this is without the fix
> mentioned above).  So, it seems like ih_fdclose or whatever isn't
> doing its job, and it seems like that warrants investigation as well.

We're pretty far into -devel territory now, I think, but...

I think this is another separate bug, though I don't know if it
manifests as an actual noticeable issue when gerrit 4272 is in play.
While we do issue IH_REALLYCLOSE for all of the vnode handles for the
volume, the handle we're leaving open isn't part of the volume that's
going offline (in my usage, anyway). This may be a bit hard to explain,
but it's something like this...

We open some vnode in RW vol for writing, and CoW it; our old handle for
inode A is closed, and write to the new handle B (the RW copy). When the
vol goes offline, handle B sticks around because of the leak fixed by
4272. The volume is released, and we write to the vnode again. We CoW it
so now the RW has a new handle C, and FDH_REALLYCLOSE's handle B.
However, handle B never goes away, since while we do IH_RELEASE it in
the CoW/StoreData code, the ref count doesn't go down to 0, since we
leaked a positive ref earlier.

In theory, this should just be a resource leak, and the handle for B
should not stay open because FDH_REALLYCLOSE is supposed to close the
underlying fd. However, due to CoW refactoring in 1.6, the original
target handle is opened twice. StoreData_RXStyle holds the ref in
origfdP and CopyOnWrite opens another FdHandle_t from the target vnode's
ihandle. So, when HAVE_PIO is defined, this means that when CoW
IH_OPEN's the targetvnode->handle, it gets back the same FdHandle_t as
StorData_RXStyle has, but the ref count is inc'd. And if you look
closely at fd_reallyclose, you'll see that it is effectively a no-op
when fd_refcnt > 1.

This isn't an issue when gerrit 4272 fixes the ihandle leak, since when
the ihandle refcnt reaches 0, we close all associated file descriptors

A conservative fix for this I think would be to add some field to
FdHandle_t to indicate that a reallyclose is needed, so a subsequent
call to FDH_CLOSE causes an effective FDH_REALLYCLOSE. (Like how
ihandle_t has the IH_REALLY_CLOSED flag.) A "better" fix would be to add
a new fd_status type, but I'd be worried for stable 1.6.x about screwing
up the other fd_status checks.

Andrew Deason