Re: "simultaneous" mounts causing weird behavior

[Matthew Mitchell <matthew@geodev.com>]

On Tue, 2003-11-04 at 11:02, H. Peter Anvin wrote:
> Matthew Mitchell wrote:
> > Hello,
> > 
> > On some SMP processing nodes we have in our cluster we are noticing the
> > following odd behavior.  It seems like there might be a race condition
> > somewhere in automount that results in the same (in this case NFS)
> > device mounted twice on the same mountpoint.
> > 
> > In our case we have a (closed-source, vendor provided) data processing
> > app that runs 2-4 processes at a time on each of these nodes.  The
> > processes communicate via MPI.  What ends up happening is that each of
> > them tries to read data from these NFS-mounted volumes at exactly the
> > same time, and sometimes (about one node out of every 10) we get unlucky
> > and the disk gets double-mounted.
> > 
> > Here is the entry from the messages file where the disks are getting
> > mounted:
> > Nov  2 16:52:53 fir32 automount[674]: attempting to mount entry
> > /etvf/data0
> > Nov  2 16:52:53 fir32 automount[674]: attempting to mount entry
> > /etvf/data0
> > 
> > (Yes, there are two of them.)
> > 
> 
> This happens because mount silently changed behaviour -- autofs relies 
> on mount only allowing one thing to be mounted on each mount point, but 
> that was suddenly changed without warning.

Yes, I know that mount can do that now.  But automount is spawning the
second mount before the first one completes, no?  Still seems like there
should be some sort of mutual exclusion around the lookup_mount call.  I
realize our workload is kind of funny in that the processes effectively
_try_ to synchronize their automount request.  

Has any of this changed in the 4.0-pre autofs?  I haven't read through
its source yet.

-m