Re: umount() and NFS races in 2.4.26

[raven]

On Sat, 10 Jul 2004, Thomas Moestl wrote:

> Hello,
> 
> On Sat, 2004/07/10 at 14:57:46 +0800, raven@xxxxxxxxxx wrote:
> > > Hi,
> > > 
> > > after deploying an SMP machine at work, we started to experience Oopses
> > > in file-system related code relatively frequently. Investigation
> > > revealed that they were caused by references using junk pointers from
> > > freed super blocks via dangling inodes from unmounted file systems;
> > > Oopses would always be preceded by the warning
> > >   VFS: Busy inodes after unmount. Self-destruct in 5 seconds.  Have a nice day...
> > > on an unmount (unmount activity is high on this machine due to heavy use
> > > of the automounter). The predecessor to this machine, a UP system with
> > > otherwise almost identical configuration, did never encounter such
> > > problems, so I went looking for possible SMP races.
> > 
> > This has been reported many times by users of autofs, especially people 
> > with a ot of mount/umount activity.
> > 
> > As James pointed out my latest autofs4 patch resolved the issue for him.
> > However, on the NFS list Greg Banks pointed out that this may be hiding a
> > problem that exists in NFS. So it would be good if the NFS folk could 
> > investigate this further.
> > 
> > Never the less I'm sure there is a race in waitq.c of autofs4 in 
> > 2.4 that seems to cause this problem. This is one of the things 
> > addressed by my patch.
> 
> The system in question still uses autofs3. While I believe that the
> waitq race is also present there (it could probably cause directory
> lookups to hang, if I understand it correctly), I do not think that
> any autofs3 code could cause exactly those symptoms that I have
> observed. For that, it would have to obtain dentries of the file
> systems that it has mounted, but the old code never does that.

All autofs has to do is not delete a directory before exiting for this 
error to occur.

Ian

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