Re: [Qemu-devel] [PATCH 0/3] fs/9p: fix setattr/getattr issues with open files

[Greg Kurz]

On Thu, 7 Jul 2016 15:34:34 +0200
Greg Kurz <address@hidden> wrote:

> On Thu, 7 Jul 2016 14:35:40 +0200
> Dominique Martinet <address@hidden> 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 <address@hidden> 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.
> 

The error indeed comes from v9fs_file_open()->v9fs_fid_clone(). I'll
try to find a fix next week.

Cheers.

--
Greg


> > 
> > 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