On Tue, Oct 23, 2012 at 06:36:15PM +0100, Nix wrote:
> On 23 Oct 2012, nix@xxxxxxxxxxxxx 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;
> }
>
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