rhashtable: Fix sleeping inside RCU critical section in walk_stop

The commit 963ecbd41a1026d99ec7537c050867428c397b89 ("rhashtable:
Fix use-after-free in rhashtable_walk_stop") fixed a real bug
but created another one because we may end up sleeping inside an
RCU critical section.

This patch fixes it properly by replacing the mutex with a spin
lock that specifically protects the walker lists.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 7 insertions, 2 deletions

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index f9ecf32bce55..d7be9cb0e91f 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -133,6 +133,7 @@ struct rhashtable_params {
  * @p: Configuration parameters
  * @run_work: Deferred worker to expand/shrink asynchronously
  * @mutex: Mutex to protect current/future table swapping
+ * @lock: Spin lock to protect walker list
  * @being_destroyed: True if table is set up for destruction
  */
 struct rhashtable {
@@ -144,6 +145,7 @@ struct rhashtable {
         struct rhashtable_params        p;
         struct work_struct              run_work;
         struct mutex                    mutex;
+        spinlock_t                      lock;
 };
 
 /**
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 7686c1e9934a..e96ad1a52c90 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -256,8 +256,10 @@ static int rhashtable_rehash_table(struct rhashtable *ht)
         /* Publish the new table pointer. */
         rcu_assign_pointer(ht->tbl, new_tbl);
 
+        spin_lock(&ht->lock);
         list_for_each_entry(walker, &old_tbl->walkers, list)
                 walker->tbl = NULL;
+        spin_unlock(&ht->lock);
 
         /* Wait for readers. All new readers will see the new
          * table, and thus no references to the old table will
@@ -635,12 +637,12 @@ void rhashtable_walk_stop(struct rhashtable_iter *iter)
 
         ht = iter->ht;
 
-        mutex_lock(&ht->mutex);
+        spin_lock(&ht->lock);
         if (tbl->rehash < tbl->size)
                 list_add(&iter->walker->list, &tbl->walkers);
         else
                 iter->walker->tbl = NULL;
-        mutex_unlock(&ht->mutex);
+        spin_unlock(&ht->lock);
 
         iter->p = NULL;
 
@@ -723,6 +725,7 @@ int rhashtable_init(struct rhashtable *ht,
 
         memset(ht, 0, sizeof(*ht));
         mutex_init(&ht->mutex);
+        spin_lock_init(&ht->lock);
         memcpy(&ht->p, params, sizeof(*params));
 
         if (params->min_size)