On Thu, 10 Jul 2014 11:59:05 -0400
"J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> On Tue, Jul 08, 2014 at 02:02:49PM -0400, Jeff Layton wrote:
> > Bruce says:
> >
> > There's also a preexisting expire_client/laundromat vs break race:
> >
> > - expire_client/laundromat adds a delegation to its local
> > reaplist using the same dl_recall_lru field that a delegation
> > uses to track its position on the recall lru and drops the
> > state lock.
> >
> > - a concurrent break_lease adds the delegation to the lru.
> >
> > - expire/client/laundromat then walks it reaplist and sees the
> > lru head as just another delegation on the list....
> >
> > Fix this race by checking the dl_time under the state_lock. If we find
> > that it's not 0, then we know that it has already been queued to the LRU
> > list and that we shouldn't queue it again.
> >
> > In the case of destroy_client, we must also ensure that we don't hit
> > similar races by ensuring that we don't move any delegations to the
> > reaplist with a dl_time of 0. Just bump the dl_time by one before we
> > drop the state_lock. We're destroying the delegations anyway, so a 1s
> > difference there won't matter.
> >
> > The fault injection code also requires a bit of surgery here:
> >
> > First, in the case of nfsd_forget_client_delegations, we must prevent
> > the same sort of race vs. the delegation break callback. For that, we
> > just increment the dl_time to ensure that a delegation callback can't
> > race in while we're working on it.
> >
> > We can't do that for nfsd_recall_client_delegations, as we need to have
> > it actually queue the delegation, and that won't happen if we increment
> > the dl_time. The state lock is held over that function, so we don't need
> > to worry about these sorts of races there.
> >
> > There is one other potential bug nfsd_recall_client_delegations though.
> > Entries on the victims list are not dequeued before calling
> > nfsd_break_one_deleg. That's a potential list corruptor, so ensure that
> > we do that there.
> >
> > Reported-by: "J. Bruce Fields" <bfields@xxxxxxxxxxxx>
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
> > ---
> > fs/nfsd/nfs4state.c | 40 +++++++++++++++++++++++++++++++++-------
> > 1 file changed, 33 insertions(+), 7 deletions(-)
> >
> > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > index 763aeeb67ccf..633b34fd6c92 100644
> > --- a/fs/nfsd/nfs4state.c
> > +++ b/fs/nfsd/nfs4state.c
> > @@ -1287,6 +1287,8 @@ destroy_client(struct nfs4_client *clp)
> > while (!list_empty(&clp->cl_delegations)) {
> > dp = list_entry(clp->cl_delegations.next, struct nfs4_delegation, dl_perclnt);
> > list_del_init(&dp->dl_perclnt);
> > + /* Ensure that deleg break won't try to requeue it */
> > + ++dp->dl_time;
> > list_move(&dp->dl_recall_lru, &reaplist);
> > }
> > spin_unlock(&state_lock);
> > @@ -2933,10 +2935,14 @@ static void nfsd_break_one_deleg(struct nfs4_delegation *dp)
> > * it's safe to take a reference: */
> > atomic_inc(&dp->dl_count);
> >
> > - list_add_tail(&dp->dl_recall_lru, &nn->del_recall_lru);
> > -
> > - /* Only place dl_time is set; protected by i_lock: */
> > - dp->dl_time = get_seconds();
> > + /*
> > + * If the dl_time != 0, then we know that it has already been
> > + * queued for a lease break. Don't queue it again.
> > + */
> > + if (dp->dl_time == 0) {
>
> Any reason not to just make that
>
> if (dp->dl_time)
> return;
>
> ?
>
> I don't know if it matters right now, but it might also be useful to
> know that no more work will get queued once dl_time is set.
>
> --b.
>
No, that makes a lot of sense. I'll respin and do that instead...
> > + list_add_tail(&dp->dl_recall_lru, &nn->del_recall_lru);
> > + dp->dl_time = get_seconds();
> > + }
> >
> > block_delegations(&dp->dl_fh);
> >
> > @@ -5081,8 +5087,23 @@ static u64 nfsd_find_all_delegations(struct nfs4_client *clp, u64 max,
> >
> > lockdep_assert_held(&state_lock);
> > list_for_each_entry_safe(dp, next, &clp->cl_delegations, dl_perclnt) {
> > - if (victims)
> > + if (victims) {
> > + /*
> > + * It's not safe to mess with delegations that have a
> > + * non-zero dl_time. They might have already been broken
> > + * and could be processed by the laundromat outside of
> > + * the state_lock. Just leave them be.
> > + */
> > + if (dp->dl_time != 0)
> > + continue;
> > +
> > + /*
> > + * Increment dl_time to ensure that delegation breaks
> > + * don't monkey with it now that we are.
> > + */
> > + ++dp->dl_time;
> > list_move(&dp->dl_recall_lru, victims);
> > + }
> > if (++count == max)
> > break;
> > }
> > @@ -5107,14 +5128,19 @@ u64 nfsd_forget_client_delegations(struct nfs4_client *clp, u64 max)
> >
> > u64 nfsd_recall_client_delegations(struct nfs4_client *clp, u64 max)
> > {
> > - struct nfs4_delegation *dp, *next;
> > + struct nfs4_delegation *dp;
> > LIST_HEAD(victims);
> > u64 count;
> >
> > spin_lock(&state_lock);
> > count = nfsd_find_all_delegations(clp, max, &victims);
> > - list_for_each_entry_safe(dp, next, &victims, dl_recall_lru)
> > + while (!list_empty(&victims)) {
> > + dp = list_first_entry(&victims, struct nfs4_delegation,
> > + dl_recall_lru);
> > + list_del_init(&dp->dl_recall_lru);
> > + dp->dl_time = 0;
> > nfsd_break_one_deleg(dp);
> > + }
> > spin_unlock(&state_lock);
> >
> > return count;
> > --
> > 1.9.3
> >
--
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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