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rhashtable: Fix walker behaviour during rehash
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Previously whenever the walker encountered a resize it simply
snaps back to the beginning and starts again.  However, this only
works if the rehash started and completed while the walker was
idle.

If the walker attempts to restart while the rehash is still ongoing,
we may miss objects that we shouldn't have.

This patch fixes this by making the walker walk the old table
followed by the new table just like all other readers.  If a
rehash is detected we will still signal our caller of the fact
so they can prepare for duplicates but we will simply continue
the walk onto the new table after the old one is finished either
by us or by the rehasher.

Signed-off-by: Herbert Xu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
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@herbertx @davem330
herbertx authored and davem330 committed Mar 15, 2015
1 parent 96026d0 commit eddee5b
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Showing 2 changed files with 50 additions and 27 deletions.
8 changes: 4 additions & 4 deletions include/linux/rhashtable.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -53,6 +53,7 @@ struct rhash_head {
* @shift: Current size (1 << shift)
* @locks_mask: Mask to apply before accessing locks[]
* @locks: Array of spinlocks protecting individual buckets
* @walkers: List of active walkers
* @buckets: size * hash buckets
*/
struct bucket_table {
Expand All @@ -61,6 +62,7 @@ struct bucket_table {
u32 shift;
unsigned int locks_mask;
spinlock_t *locks;
struct list_head walkers;

struct rhash_head __rcu *buckets[] ____cacheline_aligned_in_smp;
};
Expand Down Expand Up @@ -104,7 +106,6 @@ struct rhashtable_params {
* @p: Configuration parameters
* @run_work: Deferred worker to expand/shrink asynchronously
* @mutex: Mutex to protect current/future table swapping
* @walkers: List of active walkers
* @being_destroyed: True if table is set up for destruction
*/
struct rhashtable {
Expand All @@ -115,17 +116,16 @@ struct rhashtable {
struct rhashtable_params p;
struct work_struct run_work;
struct mutex mutex;
struct list_head walkers;
};

/**
* struct rhashtable_walker - Hash table walker
* @list: List entry on list of walkers
* @resize: Resize event occured
* @tbl: The table that we were walking over
*/
struct rhashtable_walker {
struct list_head list;
bool resize;
struct bucket_table *tbl;
};

/**
Expand Down
69 changes: 46 additions & 23 deletions lib/rhashtable.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -170,6 +170,8 @@ static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
return NULL;
}

INIT_LIST_HEAD(&tbl->walkers);

for (i = 0; i < nbuckets; i++)
INIT_RHT_NULLS_HEAD(tbl->buckets[i], ht, i);

Expand Down Expand Up @@ -264,6 +266,7 @@ static void rhashtable_rehash(struct rhashtable *ht,
struct bucket_table *new_tbl)
{
struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
struct rhashtable_walker *walker;
unsigned old_hash;

get_random_bytes(&new_tbl->hash_rnd, sizeof(new_tbl->hash_rnd));
Expand All @@ -284,6 +287,9 @@ static void rhashtable_rehash(struct rhashtable *ht,
/* Publish the new table pointer. */
rcu_assign_pointer(ht->tbl, new_tbl);

list_for_each_entry(walker, &old_tbl->walkers, list)
walker->tbl = NULL;

/* Wait for readers. All new readers will see the new
* table, and thus no references to the old table will
* remain.
Expand Down Expand Up @@ -358,7 +364,6 @@ static void rht_deferred_worker(struct work_struct *work)
{
struct rhashtable *ht;
struct bucket_table *tbl;
struct rhashtable_walker *walker;

ht = container_of(work, struct rhashtable, run_work);
mutex_lock(&ht->mutex);
Expand All @@ -367,9 +372,6 @@ static void rht_deferred_worker(struct work_struct *work)

tbl = rht_dereference(ht->tbl, ht);

list_for_each_entry(walker, &ht->walkers, list)
walker->resize = true;

if (rht_grow_above_75(ht, tbl))
rhashtable_expand(ht);
else if (rht_shrink_below_30(ht, tbl))
Expand Down Expand Up @@ -725,11 +727,9 @@ int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter)
if (!iter->walker)
return -ENOMEM;

INIT_LIST_HEAD(&iter->walker->list);
iter->walker->resize = false;

mutex_lock(&ht->mutex);
list_add(&iter->walker->list, &ht->walkers);
iter->walker->tbl = rht_dereference(ht->tbl, ht);
list_add(&iter->walker->list, &iter->walker->tbl->walkers);
mutex_unlock(&ht->mutex);

return 0;
Expand All @@ -745,7 +745,8 @@ EXPORT_SYMBOL_GPL(rhashtable_walk_init);
void rhashtable_walk_exit(struct rhashtable_iter *iter)
{
mutex_lock(&iter->ht->mutex);
list_del(&iter->walker->list);
if (iter->walker->tbl)
list_del(&iter->walker->list);
mutex_unlock(&iter->ht->mutex);
kfree(iter->walker);
}
Expand All @@ -767,12 +768,19 @@ EXPORT_SYMBOL_GPL(rhashtable_walk_exit);
*/
int rhashtable_walk_start(struct rhashtable_iter *iter)
{
struct rhashtable *ht = iter->ht;

mutex_lock(&ht->mutex);

if (iter->walker->tbl)
list_del(&iter->walker->list);

rcu_read_lock();

if (iter->walker->resize) {
iter->slot = 0;
iter->skip = 0;
iter->walker->resize = false;
mutex_unlock(&ht->mutex);

if (!iter->walker->tbl) {
iter->walker->tbl = rht_dereference_rcu(ht->tbl, ht);
return -EAGAIN;
}

Expand All @@ -794,13 +802,11 @@ EXPORT_SYMBOL_GPL(rhashtable_walk_start);
*/
void *rhashtable_walk_next(struct rhashtable_iter *iter)
{
const struct bucket_table *tbl;
struct bucket_table *tbl = iter->walker->tbl;
struct rhashtable *ht = iter->ht;
struct rhash_head *p = iter->p;
void *obj = NULL;

tbl = rht_dereference_rcu(ht->tbl, ht);

if (p) {
p = rht_dereference_bucket_rcu(p->next, tbl, iter->slot);
goto next;
Expand All @@ -826,17 +832,18 @@ void *rhashtable_walk_next(struct rhashtable_iter *iter)
iter->skip = 0;
}

iter->p = NULL;

out:
if (iter->walker->resize) {
iter->p = NULL;
iter->walker->tbl = rht_dereference_rcu(ht->future_tbl, ht);
if (iter->walker->tbl != tbl) {
iter->slot = 0;
iter->skip = 0;
iter->walker->resize = false;
return ERR_PTR(-EAGAIN);
}

iter->walker->tbl = NULL;
iter->p = NULL;

out:

return obj;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_next);
Expand All @@ -849,7 +856,24 @@ EXPORT_SYMBOL_GPL(rhashtable_walk_next);
*/
void rhashtable_walk_stop(struct rhashtable_iter *iter)
{
struct rhashtable *ht;
struct bucket_table *tbl = iter->walker->tbl;

rcu_read_unlock();

if (!tbl)
return;

ht = iter->ht;

mutex_lock(&ht->mutex);
if (rht_dereference(ht->tbl, ht) == tbl ||
rht_dereference(ht->future_tbl, ht) == tbl)
list_add(&iter->walker->list, &tbl->walkers);
else
iter->walker->tbl = NULL;
mutex_unlock(&ht->mutex);

iter->p = NULL;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_stop);
Expand Down Expand Up @@ -927,7 +951,6 @@ int rhashtable_init(struct rhashtable *ht, struct rhashtable_params *params)
memset(ht, 0, sizeof(*ht));
mutex_init(&ht->mutex);
memcpy(&ht->p, params, sizeof(*params));
INIT_LIST_HEAD(&ht->walkers);

if (params->locks_mul)
ht->p.locks_mul = roundup_pow_of_two(params->locks_mul);
Expand Down

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