[PATCH 6/8] rhashtable: further improve stability of rhashtable_walk

NeilBrown
2018-05-04T03:55:21Z

If the sequence:
obj = rhashtable_walk_next(iter);
rhashtable_walk_stop(iter);
rhashtable_remove_fast(ht, &obj->head, params);
rhashtable_walk_start(iter);

races with another thread inserting or removing
an object on the same hash chain, a subsequent
rhashtable_walk_next() is not guaranteed to get the "next"
object. It is possible that an object could be
repeated, or missed.

This can be made more reliable by keeping the objects in a hash chain
sorted by memory address. A subsequent rhashtable_walk_next()
call can reliably find the correct position in the list, and thus
find the 'next' object.

It is not possible (certainly not so easy) to achieve this with an
rhltable as keeping the hash chain in order is not so easy. When the
first object with a given key is removed, it is replaced in the chain
with the next object with the same key, and the address of that
object may not be correctly ordered.
No current user of rhltable_walk_enter() calls
rhashtable_walk_start() more than once, so no current code
could benefit from a more reliable walk of rhltables.

This patch only attempts to improve walks for rhashtables.
- a new object is always inserted after the last object with a
smaller address, or at the start
- when rhashtable_walk_start() is called, it records that 'p' is not
'safe', meaning that it cannot be dereferenced. The revalidation
that was previously done here is moved to rhashtable_walk_next()
- when rhashtable_walk_next() is called while p is not NULL and not
safe, it walks the chain looking for the first object with an
address greater than p and returns that. If there is none, it moves
to the next hash chain.

Signed-off-by: NeilBrown <neilb@suse.com>
---
include/linux/rhashtable.h | 11 +++++-
lib/rhashtable.c | 82 ++++++++++++++++++++++++++++----------------
2 files changed, 62 insertions(+), 31 deletions(-)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index 5091abf975a1..20684a451cb0 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -188,6 +188,7 @@ struct rhashtable_iter {
struct rhashtable_walker walker;
unsigned int slot;
unsigned int skip;
+ bool p_is_unsafe;
bool end_of_table;
};

@@ -737,7 +738,12 @@ static inline void *__rhashtable_insert_fast(
(params.obj_cmpfn ?
params.obj_cmpfn(&arg, rht_obj(ht, head)) :
rhashtable_compare(&arg, rht_obj(ht, head)))) {
- pprev = &head->next;
+ if (rhlist) {
+ pprev = &head->next;
+ } else {
+ if (head < obj)
+ pprev = &head->next;
+ }
continue;
}

@@ -1233,7 +1239,8 @@ static inline int rhashtable_walk_init(struct rhashtable *ht,
* Note that if you restart a walk after rhashtable_walk_stop you
* may see the same object twice. Also, you may miss objects if
* there are removals in between rhashtable_walk_stop and the next
- * call to rhashtable_walk_start.
+ * call to rhashtable_walk_start. Note that this is different to
+ * rhashtable_walk_enter() which misses objects.
*
* For a completely stable walk you should construct your own data
* structure outside the hash table.
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 83f5d1ebf452..038c4156b66a 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -234,6 +234,7 @@ static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
struct bucket_table *new_tbl = rhashtable_last_table(ht,
rht_dereference_rcu(old_tbl->future_tbl, ht));
struct rhash_head __rcu **pprev = rht_bucket_var(old_tbl, old_hash);
+ struct rhash_head __rcu **inspos;
int err = -EAGAIN;
struct rhash_head *head, *next, *entry;
spinlock_t *new_bucket_lock;
@@ -262,12 +263,15 @@ static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
new_bucket_lock = rht_bucket_lock(new_tbl, new_hash);

spin_lock_nested(new_bucket_lock, SINGLE_DEPTH_NESTING);
- head = rht_dereference_bucket(new_tbl->buckets[new_hash],
- new_tbl, new_hash);
-
+ inspos = &new_tbl->buckets[new_hash];
+ head = rht_dereference_bucket(*inspos, new_tbl, new_hash);
+ while (!rht_is_a_nulls(head) && head < entry) {
+ inspos = &head->next;
+ head = rht_dereference_bucket(*inspos, new_tbl, new_hash);
+ }
RCU_INIT_POINTER(entry->next, head);

- rcu_assign_pointer(new_tbl->buckets[new_hash], entry);
+ rcu_assign_pointer(*inspos, entry);
spin_unlock(new_bucket_lock);

rcu_assign_pointer(*pprev, next);
@@ -565,6 +569,10 @@ static struct bucket_table *rhashtable_insert_one(struct rhashtable *ht,
return ERR_PTR(-ENOMEM);

head = rht_dereference_bucket(*pprev, tbl, hash);
+ while (!rht_is_a_nulls(head) && head < obj) {
+ pprev = &head->next;
+ head = rht_dereference_bucket(*pprev, tbl, hash);
+ }

RCU_INIT_POINTER(obj->next, head);
if (ht->rhlist) {
@@ -659,10 +667,10 @@ EXPORT_SYMBOL_GPL(rhashtable_insert_slow);
*
* This function prepares a hash table walk.
*
- * Note that if you restart a walk after rhashtable_walk_stop you
- * may see the same object twice. Also, you may miss objects if
- * there are removals in between rhashtable_walk_stop and the next
- * call to rhashtable_walk_start.
+ * A walk is guaranteed to return every object that was in
+ * the table before this call, and is still in the table when
+ * rhashtable_walk_next() returns NULL. Duplicates can be
+ * seen, but only if there is a rehash event during the walk.
*
* For a completely stable walk you should construct your own data
* structure outside the hash table.
@@ -746,19 +754,10 @@ int rhashtable_walk_start_check(struct rhashtable_iter *iter)

if (iter->p && !rhlist) {
/*
- * We need to validate that 'p' is still in the table, and
- * if so, update 'skip'
+ * 'p' will be revalidated when rhashtable_walk_next()
+ * is called.
*/
- struct rhash_head *p;
- int skip = 0;
- rht_for_each_rcu(p, iter->walker.tbl, iter->slot) {
- skip++;
- if (p == iter->p) {
- iter->skip = skip;
- goto found;
- }
- }
- iter->p = NULL;
+ iter->p_is_unsafe = true;
} else if (iter->p && rhlist) {
/* Need to validate that 'list' is still in the table, and
* if so, update 'skip' and 'p'.
@@ -875,15 +874,39 @@ void *rhashtable_walk_next(struct rhashtable_iter *iter)
bool rhlist = ht->rhlist;

if (p) {
- if (!rhlist || !(list = rcu_dereference(list->next))) {
- p = rcu_dereference(p->next);
- list = container_of(p, struct rhlist_head, rhead);
- }
- if (!rht_is_a_nulls(p)) {
- iter->skip++;
- iter->p = p;
- iter->list = list;
- return rht_obj(ht, rhlist ? &list->rhead : p);
+ if (!rhlist && iter->p_is_unsafe) {
+ /*
+ * First time next() was called after start().
+ * Need to find location of 'p' in the list.
+ */
+ struct rhash_head *p;
+
+ iter->skip = 0;
+ rht_for_each_rcu(p, iter->walker.tbl, iter->slot) {
+ iter->skip++;
+ if (p <= iter->p)
+ continue;
+
+ /* p is the next object after iter->p */
+ iter->p = p;
+ iter->p_is_unsafe = false;
+ return rht_obj(ht, p);
+ }
+ /* There is no "next" object in the list, move
+ * to next hash chain.
+ */
+ } else {
+ if (!rhlist || !(list = rcu_dereference(list->next))) {
+ p = rcu_dereference(p->next);
+ list = container_of(p, struct rhlist_head,
+ rhead);
+ }
+ if (!rht_is_a_nulls(p)) {
+ iter->skip++;
+ iter->p = p;
+ iter->list = list;
+ return rht_obj(ht, rhlist ? &list->rhead : p);
+ }
}

/* At the end of this slot, switch to next one and then find
@@ -893,6 +916,7 @@ void *rhashtable_walk_next(struct rhashtable_iter *iter)
iter->slot++;
}

+ iter->p_is_unsafe = false;
return __rhashtable_walk_find_next(iter);
}
EXPORT_SYMBOL_GPL(rhashtable_walk_next);