Re: Heads-up: 3.6.2 / 3.6.3 NFS server oops: 3.6.2+ regression? (also an unrelated ext4 data loss bug)

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Oct 23, 2012 at 06:36:15PM +0100, Nix wrote:
> On 23 Oct 2012, nix@esperi.org.uk uttered the following:
> 
> > On 23 Oct 2012, Trond Myklebust spake thusly:
> >> On Tue, 2012-10-23 at 12:46 -0400, J. Bruce Fields wrote:
> >>> Looks like there's some confusion about whether nsm_client_get() returns
> >>> NULL or an error?
> >>
> >> nsm_client_get() looks extremely racy in the case where ln->nsm_users ==
> >> 0.  Since we never recheck the value of ln->nsm_users after taking
> >> nsm_create_mutex, what is stopping 2 different threads from both setting
> >> ln->nsm_clnt and re-initialising ln->nsm_users?
> >
> > Yep. At the worst possible time:
> >
> > 	spin_lock(&ln->nsm_clnt_lock);
> > 	if (ln->nsm_users) {
> > 		if (--ln->nsm_users)
> > 			ln->nsm_clnt = NULL;
> > (1)		shutdown = !ln->nsm_users;
> > 	}
> > 	spin_unlock(&ln->nsm_clnt_lock);
> >
> > If a thread reinitializes nsm_users at point (1), after the assignment,
> > we could well end up with ln->nsm_clnt NULL and shutdown false. A bit
> > later, nsm_mon_unmon gets called with a NULL clnt, and boom.
> 
> Possible fix if so, utterly untested so far (will test when I can face
> yet another reboot and fs-corruption-recovery-hell cycle, in a few
> hours), may ruin performance, violate locking hierarchies, and consume
> kittens:

Right, mutexes can't be taken while holding spinlocks.  Keep the kittens
well away from the computer.

--b.

> 
> diff --git a/fs/lockd/mon.c b/fs/lockd/mon.c
> index e4fb3ba..da91cdf 100644
> --- a/fs/lockd/mon.c
> +++ b/fs/lockd/mon.c
> @@ -98,7 +98,6 @@ static struct rpc_clnt *nsm_client_get(struct net *net)
>  		spin_unlock(&ln->nsm_clnt_lock);
>  		goto out;
>  	}
> -	spin_unlock(&ln->nsm_clnt_lock);
>  
>  	mutex_lock(&nsm_create_mutex);
>  	clnt = nsm_create(net);
> @@ -108,6 +107,7 @@ static struct rpc_clnt *nsm_client_get(struct net *net)
>  		ln->nsm_users = 1;
>  	}
>  	mutex_unlock(&nsm_create_mutex);
> +	spin_unlock(&ln->nsm_clnt_lock);
>  out:
>  	return clnt;
>  }
>