Re: "simultaneous" mounts causing weird behavior

["H. Peter Anvin" <hpa@zytor.com>]

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.

	-hpa