> -----Original Message----- > From: Nix [mailto:n...@esperi.org.uk] > Sent: Tuesday, October 23, 2012 1:36 PM > To: Myklebust, Trond > Cc: J. Bruce Fields; Ted Ts'o; linux-kernel@vger.kernel.org; Schumaker, > Bryan; Peng Tao; gre...@linuxfoundation.org; linux-...@vger.kernel.org; > Stanislav Kinsbursky > Subject: Re: Heads-up: 3.6.2 / 3.6.3 NFS server oops: 3.6.2+ regression? (also > an unrelated ext4 data loss bug) > > On 23 Oct 2012, n...@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: > > 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);
You can't hold a spinlock while sleeping. Both mutex_lock() and nsm_create() can definitely sleep. The correct way to do this is to grab the spinlock and recheck the value of ln->nsm_users inside the 'if (!IS_ERR())' condition. If it is still zero, bump it and set ln->nsm_clnt, otherwise bump it, get the existing ln->nsm_clnt and call rpc_shutdown_clnt() on the redundant nsm client after dropping the spinlock. Cheers Trond -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
