Re: [RFC PATCH]autofs4: hang and proposed fix

[Trond Myklebust <trond.myklebust@fys.uio.no>]

On Wed, 2005-11-30 at 08:49 -0800, Badari Pulavarty wrote:
> On Wed, 2005-11-30 at 09:02 -0500, Ian Kent wrote:
> > On Tue, 29 Nov 2005, William H. Taber wrote:
> > 
> > > Ian Kent wrote:
> > > > We'll need to do an analysis of all callers of the revalidate method.
> > > You are right. Searching through the sources, it would appear that I 
> > > missed fixing autofs and devfs.  Everyone else just defines a revalidate 
> > > routine but doesn't call one.  You may find devfs to be interesting 
> > > because they have code to determine whether they need to release the 
> > > i_sem lock or not.  I am working on an updated patch to include the 
> > > changes needed for these two modules.
> > 
> > I've looked at devfs before but that bit of code sounds interesting to me.
> > 
> > The other thing that concerns me is that we may be increasing the latency 
> > of some code paths that need to be really fast. I was thinking that 
> > perhaps it might be good to try a change more in line with the locking 
> > used in link_patch_walk (ie. i_sem free revalidate) rather than that used 
> > in lookup_one_len. My only justification being that lookup is called to 
> > create stuff where revalidate is called to check stuff. I've been 
> > poking around and this change looks fairly difficult as well (I seem to 
> > remember you also looked at this).
> > 
> > Anyway, I'm keen to have a look at your patch.
> > Thanks much for your interest and help.
> > 
> > Ian
> > 
> 
> Again, I am posting Will's latest patch on his behalf.
> 
> Any thoughts on how acceptable are the VFS changes ?

That will slow link_path_walk() for commonly accessed shared directories
(/lib, /usr/share,...) down to a crawl.

Instead of having lock-free lookups of cached dentries, you are suddenly
serialising everybody in the parent directory.

Cheers,
  Trond