nfsd: close potential race between delegation break and laundromat
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@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 324e80f..63c1420 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1288,6 +1288,8 @@
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);
@@ -2935,10 +2937,14 @@
* 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) {
+ list_add_tail(&dp->dl_recall_lru, &nn->del_recall_lru);
+ dp->dl_time = get_seconds();
+ }
block_delegations(&dp->dl_fh);
@@ -5083,8 +5089,23 @@
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;
}
@@ -5109,14 +5130,19 @@
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;