Re: [NFS] RE: [autofs] multiple servers per automount

[Ian Kent]

Please forgive my ignorance Ingo but ...

I suffer from race condition blindness. A terible afflicition when one is 
trying to understand the sublties of the kernel, but I'm trying.

While I am not questioning your suggestion, I have thought about the code 
and fail to see the race you point out. Please help me along.

On Wed, 15 Oct 2003, Ingo Oeser wrote:

> On Wednesday 15 October 2003 01:12, Mike Waychison wrote:
> > The problem still remains in 2.6 that we limit the count to 256.  I've
> > attached a quick patch that I've compiled and tested.  I don't know if
> > there is a better way to handle dynamic assignment of minors (haven't
> > kept up to date in that realm), but if there is, then we should probably
> >   use it instead.
> 
> 
> In your patch you allocate inside the spinlock.

Do you mean we don't want to sleep under the spin lock?
Would a GFP_ATOMIC make a difference to the analysis?

> 
> I would suggest to do sth. like the following:
> 
> void *local;
> if (!unamed_dev_inuse) {
>     local = get_zeroed_page(GFP_KERNEL);
> 
>     if (!local) 
>         return -ENOMEM;
> }
> 
> spinlock(&unamed_dev_lock);
> mb();
> if (!unamed_dev_inuse) {
>     unamed_dev_inuse = local;
> 
>     /* Used globally, don't free now */
>     local = NULL;
> }
> 
> /* 
>   Do the lookup and alloc
>  */
> 
> spinunlock(&unamed_dev_lock);
> 
> /* Free page, because of race on allocation. */
> if (local) 
>     free_page(local);
> 
> 
> Which will swap the pointers atomically and still alloc outside the
> non-sleeping locking.

As I said please give me a hint about your thinking here.
And the use of a memory barrier as well ... umm?

-- 

   ,-._|\    Ian Kent
  /      \   Perth, Western Australia
  *_.--._/   E-mail: raven@xxxxxxxxxx
        v    Web: http://themaw.net/

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