Re: [PATCH 0/3] fs/9p: fix setattr/getattr issues with open files
From: Greg Kurz
Date: Thu Jul 07 2016 - 10:25:21 EST
On Thu, 7 Jul 2016 14:35:40 +0200
Dominique Martinet <dominique.martinet@xxxxxx> wrote:
> Hi Greg,
>
Hi Dominique,
> Greg Kurz wrote on Mon, Jul 04, 2016 at 05:08:49PM +0200:
> > On Mon, 4 Jul 2016 16:16:55 +0200
> > Dominique Martinet <dominique.martinet@xxxxxx> wrote:
> >
> > > I *think* this introduces a race somewhere, I'm getting errors like:
> > > cat: f.05: No such file or directory
> > > cat: f.14: No such file or directory
> > > cat: f.13: No such file or directory
> > > cat: f.39: No such file or directory
> > > cat: f.05: No such file or directory
> > >
> > >
> > > when doing:
> > > for file in {01..50}; do touch f.${file}; done
> > > seq 1 1000 | xargs -n 1 -P 25 -I{} cat f.* > /dev/null
>
> Ok so, tested with the first two patches and I can't seem to hit any
> problem with the qemu server at least (I'd need more time to fix
> ganesha's 9p tcp/rdma server before I could blame the client in any way)
>
I'm not surprised: patch 1 simply adds a "fallback" lookup to the existing code,
and patch 2 changes this "fallback" lookup only.
Bad things can come with patch 3 because it really changes the lookup logic.
>
> The last patch looks good to me, I think it only makes an existing race
> more visible... What I think could happen is:
> process 1 has file open
> process 2 tries to open file, sees fid open
> process 1 closes file/clunk fids
> process 2 tries to clone now-clunked fid and gets ENOENT
>
I'll try to have a look with this scenario in mind.
>
> I'm afraid I just found out my hypervisor is no longer recent enough for
> gdb kernel scripts (gdb 7.6 and python 2.7.5 in el7 compared to the
> apparently required 7.7 and 2.7.6 respectively...), and I don't see
> anything obvious with just debug messages/adding a few printks (wasn't
> able to confirm where exactly that ENOENT comes from or if my theory is
> even close to the truth)
>
> I'd like to spend more time on it but don't think I'll be able to for a
> couple of weeks ; sorry about that.
>
No problem. My plate is full anyway until I go into a 1-month vacation,
starting end of July. And I'm currently targeting QEMU 2.8 for the
server side fixes: we have plenty of time to fix this.
>
> Were you able to reproduce the problem?
>
Yes ! I get it every time :)
> Thanks,
I really appreciate your assistance since v9fs-devel is really quiet these
days.
Cheers.
--
Greg