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Subject[PATCH 3/5] rhashtable: move dereference inside rht_ptr()
Rather than dereferencing a pointer to a bucket and then passing the
result to rht_ptr(), we now pass in the pointer and do the dereference
in rht_ptr().

This requires that we pass in the tbl and hash as well to support RCU
checks, and means that the various rht_for_each functions can expect a
pointer that can be dereferenced without further care.

There are two places where we dereference a bucket pointer
where there is no testable protection - in each case we know
that we much have exclusive access without having taken a lock.
The previous code used rht_dereference() to pretend that holding
the mutex provided protects, but holding the mutex never provides
protection for accessing buckets.

So instead introduce rht_ptr_exclusive() that can be used when
there is known to be exclusive access without holding any locks.

Signed-off-by: NeilBrown <neilb@suse.com>
---
include/linux/rhashtable.h | 69 +++++++++++++++++++++++++++-----------------
lib/rhashtable.c | 12 ++++----
lib/test_rhashtable.c | 2 +
3 files changed, 49 insertions(+), 34 deletions(-)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index c504cd820736..b54e6436547e 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -344,12 +344,28 @@ static inline void rht_unlock(struct bucket_table *tbl,
}

/*
- * If 'p' is a bucket head and might be locked:
- * rht_ptr() returns the address without the lock bit.
- * rht_ptr_locked() returns the address WITH the lock bit.
+ * Where 'bkt' is a bucket and might be locked:
+ * rht_ptr() dereferences that pointer and clears the lock bit.
+ * rht_ptr_exclusive() dereferences in a context where exclusive
+ * access is guaranteed, such as when destroying the table.
*/
-static inline struct rhash_head __rcu *rht_ptr(const struct rhash_lock_head *p)
+static inline struct rhash_head *rht_ptr(
+ struct rhash_lock_head __rcu * const *bkt,
+ struct bucket_table *tbl,
+ unsigned int hash)
{
+ const struct rhash_lock_head *p =
+ rht_dereference_bucket_rcu(*bkt, tbl, hash);
+
+ return (void *)(((unsigned long)p) & ~BIT(1));
+}
+
+static inline struct rhash_head *rht_ptr_exclusive(
+ struct rhash_lock_head __rcu * const *bkt)
+{
+ const struct rhash_lock_head *p =
+ rcu_dereference_protected(*bkt, 1);
+
return (void *)(((unsigned long)p) & ~BIT(1));
}

@@ -380,8 +396,8 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
* @hash: the hash value / bucket index
*/
#define rht_for_each_from(pos, head, tbl, hash) \
- for (pos = rht_dereference_bucket(head, tbl, hash); \
- !rht_is_a_nulls(pos); \
+ for (pos = head; \
+ !rht_is_a_nulls(pos); \
pos = rht_dereference_bucket((pos)->next, tbl, hash))

/**
@@ -391,7 +407,8 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
* @hash: the hash value / bucket index
*/
#define rht_for_each(pos, tbl, hash) \
- rht_for_each_from(pos, rht_ptr(*rht_bucket(tbl, hash)), tbl, hash)
+ rht_for_each_from(pos, rht_ptr(rht_bucket(tbl, hash), tbl, hash), \
+ tbl, hash)

/**
* rht_for_each_entry_from - iterate over hash chain from given head
@@ -403,7 +420,7 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
* @member: name of the &struct rhash_head within the hashable struct.
*/
#define rht_for_each_entry_from(tpos, pos, head, tbl, hash, member) \
- for (pos = rht_dereference_bucket(head, tbl, hash); \
+ for (pos = head; \
(!rht_is_a_nulls(pos)) && rht_entry(tpos, pos, member); \
pos = rht_dereference_bucket((pos)->next, tbl, hash))

@@ -416,8 +433,9 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
* @member: name of the &struct rhash_head within the hashable struct.
*/
#define rht_for_each_entry(tpos, pos, tbl, hash, member) \
- rht_for_each_entry_from(tpos, pos, rht_ptr(*rht_bucket(tbl, hash)), \
- tbl, hash, member)
+ rht_for_each_entry_from(tpos, pos, \
+ rht_ptr(rht_bucket(tbl, hash), tbl, hash), \
+ tbl, hash, member)

/**
* rht_for_each_entry_safe - safely iterate over hash chain of given type
@@ -432,8 +450,7 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
* remove the loop cursor from the list.
*/
#define rht_for_each_entry_safe(tpos, pos, next, tbl, hash, member) \
- for (pos = rht_dereference_bucket(rht_ptr(*rht_bucket(tbl, hash)), \
- tbl, hash), \
+ for (pos = rht_ptr(rht_bucket(tbl, hash), tbl, hash), \
next = !rht_is_a_nulls(pos) ? \
rht_dereference_bucket(pos->next, tbl, hash) : NULL; \
(!rht_is_a_nulls(pos)) && rht_entry(tpos, pos, member); \
@@ -454,7 +471,7 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
*/
#define rht_for_each_rcu_from(pos, head, tbl, hash) \
for (({barrier(); }), \
- pos = rht_dereference_bucket_rcu(head, tbl, hash); \
+ pos = head; \
!rht_is_a_nulls(pos); \
pos = rcu_dereference_raw(pos->next))

@@ -469,10 +486,9 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
* traversal is guarded by rcu_read_lock().
*/
#define rht_for_each_rcu(pos, tbl, hash) \
- for (({barrier(); }), \
- pos = rht_ptr(rht_dereference_bucket_rcu( \
- *rht_bucket(tbl, hash), tbl, hash)); \
- !rht_is_a_nulls(pos); \
+ for (({barrier(); }), \
+ pos = rht_ptr(rht_bucket(tbl, hash), tbl, hash); \
+ !rht_is_a_nulls(pos); \
pos = rcu_dereference_raw(pos->next))

/**
@@ -490,7 +506,7 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
*/
#define rht_for_each_entry_rcu_from(tpos, pos, head, tbl, hash, member) \
for (({barrier(); }), \
- pos = rht_dereference_bucket_rcu(head, tbl, hash); \
+ pos = head; \
(!rht_is_a_nulls(pos)) && rht_entry(tpos, pos, member); \
pos = rht_dereference_bucket_rcu(pos->next, tbl, hash))

@@ -508,8 +524,9 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
*/
#define rht_for_each_entry_rcu(tpos, pos, tbl, hash, member) \
rht_for_each_entry_rcu_from(tpos, pos, \
- rht_ptr(*rht_bucket(tbl, hash)), \
- tbl, hash, member)
+ rht_ptr(rht_bucket(tbl, hash), \
+ tbl, hash), \
+ tbl, hash, member)

/**
* rhl_for_each_rcu - iterate over rcu hash table list
@@ -556,7 +573,6 @@ static inline struct rhash_head *__rhashtable_lookup(
};
struct rhash_lock_head __rcu * const *bkt;
struct bucket_table *tbl;
- struct rhash_head __rcu *head;
struct rhash_head *he;
unsigned int hash;

@@ -565,8 +581,7 @@ static inline struct rhash_head *__rhashtable_lookup(
hash = rht_key_hashfn(ht, tbl, key, params);
bkt = rht_bucket(tbl, hash);
do {
- head = rht_ptr(rht_dereference_bucket_rcu(*bkt, tbl, hash));
- rht_for_each_rcu_from(he, head, tbl, hash) {
+ rht_for_each_rcu_from(he, rht_ptr(bkt, tbl, hash), tbl, hash) {
if (params.obj_cmpfn ?
params.obj_cmpfn(&arg, rht_obj(ht, he)) :
rhashtable_compare(&arg, rht_obj(ht, he)))
@@ -699,7 +714,7 @@ static inline void *__rhashtable_insert_fast(
return rhashtable_insert_slow(ht, key, obj);
}

- rht_for_each_from(head, rht_ptr(*bkt), tbl, hash) {
+ rht_for_each_from(head, rht_ptr(bkt, tbl, hash), tbl, hash) {
struct rhlist_head *plist;
struct rhlist_head *list;

@@ -744,7 +759,7 @@ static inline void *__rhashtable_insert_fast(
goto slow_path;

/* Inserting at head of list makes unlocking free. */
- head = rht_ptr(rht_dereference_bucket(*bkt, tbl, hash));
+ head = rht_ptr(bkt, tbl, hash);

RCU_INIT_POINTER(obj->next, head);
if (rhlist) {
@@ -971,7 +986,7 @@ static inline int __rhashtable_remove_fast_one(
pprev = NULL;
rht_lock(tbl, bkt);

- rht_for_each_from(he, rht_ptr(*bkt), tbl, hash) {
+ rht_for_each_from(he, rht_ptr(bkt, tbl, hash), tbl, hash) {
struct rhlist_head *list;

list = container_of(he, struct rhlist_head, rhead);
@@ -1130,7 +1145,7 @@ static inline int __rhashtable_replace_fast(
pprev = NULL;
rht_lock(tbl, bkt);

- rht_for_each_from(he, rht_ptr(*bkt), tbl, hash) {
+ rht_for_each_from(he, rht_ptr(bkt, tbl, hash), tbl, hash) {
if (he != obj_old) {
pprev = &he->next;
continue;
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index ec485962de5f..1f38f550ba76 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -232,7 +232,8 @@ static int rhashtable_rehash_one(struct rhashtable *ht,

err = -ENOENT;

- rht_for_each_from(entry, rht_ptr(*bkt), old_tbl, old_hash) {
+ rht_for_each_from(entry, rht_ptr(bkt, old_tbl, old_hash),
+ old_tbl, old_hash) {
err = 0;
next = rht_dereference_bucket(entry->next, old_tbl, old_hash);

@@ -249,8 +250,7 @@ static int rhashtable_rehash_one(struct rhashtable *ht,

rht_lock_nested(new_tbl, &new_tbl->buckets[new_hash], SINGLE_DEPTH_NESTING);

- head = rht_ptr(rht_dereference_bucket(new_tbl->buckets[new_hash],
- new_tbl, new_hash));
+ head = rht_ptr(new_tbl->buckets + new_hash, new_tbl, new_hash);

RCU_INIT_POINTER(entry->next, head);

@@ -492,7 +492,7 @@ static void *rhashtable_lookup_one(struct rhashtable *ht,
int elasticity;

elasticity = RHT_ELASTICITY;
- rht_for_each_from(head, rht_ptr(*bkt), tbl, hash) {
+ rht_for_each_from(head, rht_ptr(bkt, tbl, hash), tbl, hash) {
struct rhlist_head *list;
struct rhlist_head *plist;

@@ -558,7 +558,7 @@ static struct bucket_table *rhashtable_insert_one(struct rhashtable *ht,
if (unlikely(rht_grow_above_100(ht, tbl)))
return ERR_PTR(-EAGAIN);

- head = rht_ptr(rht_dereference_bucket(*bkt, tbl, hash));
+ head = rht_ptr(bkt, tbl, hash);

RCU_INIT_POINTER(obj->next, head);
if (ht->rhlist) {
@@ -1140,7 +1140,7 @@ void rhashtable_free_and_destroy(struct rhashtable *ht,
struct rhash_head *pos, *next;

cond_resched();
- for (pos = rht_ptr(rht_dereference(*rht_bucket(tbl, i), ht)),
+ for (pos = rht_ptr_exclusive(rht_bucket(tbl, i)),
next = !rht_is_a_nulls(pos) ?
rht_dereference(pos->next, ht) : NULL;
!rht_is_a_nulls(pos);
diff --git a/lib/test_rhashtable.c b/lib/test_rhashtable.c
index 02592c2a249c..084fe5a6ac57 100644
--- a/lib/test_rhashtable.c
+++ b/lib/test_rhashtable.c
@@ -500,7 +500,7 @@ static unsigned int __init print_ht(struct rhltable *rhlt)
struct rhash_head *pos, *next;
struct test_obj_rhl *p;

- pos = rht_ptr(rht_dereference(tbl->buckets[i], ht));
+ pos = rht_ptr_exclusive(tbl->buckets + i);
next = !rht_is_a_nulls(pos) ? rht_dereference(pos->next, ht) : NULL;

if (!rht_is_a_nulls(pos)) {

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