From: Jeff Hostetler <jeffh...@microsoft.com>
This is to address concerns raised by ThreadSanitizer on the
mailing list about threaded unprotected R/W access to map.size with my previous
"disallow rehash" change (0607e10009ee4e37cb49b4cec8d28a9dda1656a4). See:
https://public-inbox.org/git/adb37b70139fd1e2bac18bfd22c8b96683ae18eb.1502780344.git.martin.ag...@gmail.com/
Add API to hashmap to disable item counting and to disable automatic rehashing.
Also include APIs to re-enable item counting and automatica rehashing.
When item counting is disabled, the map.size field is invalid. So to
prevent accidents, the field has been renamed and an accessor function
hashmap_get_size() has been added. All direct references to this
field have been been updated. And the name of the field changed
to map.private_size to communicate thie.
Signed-off-by: Jeff Hostetler <jeffh...@microsoft.com>
---
attr.c | 14 ++++---
builtin/describe.c | 2 +-
hashmap.c | 31 +++++++++++-----
hashmap.h | 98 ++++++++++++++++++++++++++++++++++++++-----------
name-hash.c | 6 ++-
t/helper/test-hashmap.c | 2 +-
6 files changed, 113 insertions(+), 40 deletions(-)
diff --git a/attr.c b/attr.c
index 56961f0..a987f37 100644
--- a/attr.c
+++ b/attr.c
@@ -149,10 +149,12 @@ struct all_attrs_item {
static void all_attrs_init(struct attr_hashmap *map, struct attr_check *check)
{
int i;
+ unsigned int size;
hashmap_lock(map);
- if (map->map.size < check->all_attrs_nr)
+ size = hashmap_get_size(&map->map);
+ if (size < check->all_attrs_nr)
die("BUG: interned attributes shouldn't be deleted");
/*
@@ -161,13 +163,13 @@ static void all_attrs_init(struct attr_hashmap *map,
struct attr_check *check)
* field), reallocate the provided attr_check instance's all_attrs
* field and fill each entry with its corresponding git_attr.
*/
- if (map->map.size != check->all_attrs_nr) {
+ if (size != check->all_attrs_nr) {
struct attr_hash_entry *e;
struct hashmap_iter iter;
hashmap_iter_init(&map->map, &iter);
- REALLOC_ARRAY(check->all_attrs, map->map.size);
- check->all_attrs_nr = map->map.size;
+ REALLOC_ARRAY(check->all_attrs, size);
+ check->all_attrs_nr = size;
while ((e = hashmap_iter_next(&iter))) {
const struct git_attr *a = e->value;
@@ -235,10 +237,10 @@ static const struct git_attr *git_attr_internal(const
char *name, int namelen)
if (!a) {
FLEX_ALLOC_MEM(a, name, name, namelen);
- a->attr_nr = g_attr_hashmap.map.size;
+ a->attr_nr = hashmap_get_size(&g_attr_hashmap.map);
attr_hashmap_add(&g_attr_hashmap, a->name, namelen, a);
- assert(a->attr_nr == (g_attr_hashmap.map.size - 1));
+ assert(a->attr_nr == (hashmap_get_size(&g_attr_hashmap.map) -
1));
}
hashmap_unlock(&g_attr_hashmap);
diff --git a/builtin/describe.c b/builtin/describe.c
index 89ea1cd..1e3cbc7 100644
--- a/builtin/describe.c
+++ b/builtin/describe.c
@@ -505,7 +505,7 @@ int cmd_describe(int argc, const char **argv, const char
*prefix)
hashmap_init(&names, (hashmap_cmp_fn) commit_name_cmp, NULL, 0);
for_each_rawref(get_name, NULL);
- if (!names.size && !always)
+ if (!hashmap_get_size(&names) && !always)
die(_("No names found, cannot describe anything."));
if (argc == 0) {
diff --git a/hashmap.c b/hashmap.c
index 9b6a128..054f334 100644
--- a/hashmap.c
+++ b/hashmap.c
@@ -116,9 +116,6 @@ static void rehash(struct hashmap *map, unsigned int
newsize)
unsigned int i, oldsize = map->tablesize;
struct hashmap_entry **oldtable = map->table;
- if (map->disallow_rehash)
- return;
-
alloc_table(map, newsize);
for (i = 0; i < oldsize; i++) {
struct hashmap_entry *e = oldtable[i];
@@ -166,6 +163,17 @@ void hashmap_init(struct hashmap *map, hashmap_cmp_fn
equals_function,
while (initial_size > size)
size <<= HASHMAP_RESIZE_BITS;
alloc_table(map, size);
+
+ /*
+ * By default, we want to:
+ * [] keep track of the number of items in the map and
+ * [] allow the map to automatically grow as necessary.
+ *
+ * We probably only need to disable these if we are being
+ * used by a threaded caller.
+ */
+ map->do_count_items = 1;
+ map->do_auto_rehash = 1;
}
void hashmap_free(struct hashmap *map, int free_entries)
@@ -206,9 +214,11 @@ void hashmap_add(struct hashmap *map, void *entry)
map->table[b] = entry;
/* fix size and rehash if appropriate */
- map->size++;
- if (map->size > map->grow_at)
- rehash(map, map->tablesize << HASHMAP_RESIZE_BITS);
+ if (map->do_count_items) {
+ map->private_size++;
+ if (map->do_auto_rehash && map->private_size > map->grow_at)
+ rehash(map, map->tablesize << HASHMAP_RESIZE_BITS);
+ }
}
void *hashmap_remove(struct hashmap *map, const void *key, const void *keydata)
@@ -224,9 +234,12 @@ void *hashmap_remove(struct hashmap *map, const void *key,
const void *keydata)
old->next = NULL;
/* fix size and rehash if appropriate */
- map->size--;
- if (map->size < map->shrink_at)
- rehash(map, map->tablesize >> HASHMAP_RESIZE_BITS);
+ if (map->do_count_items) {
+ map->private_size--;
+ if (map->do_auto_rehash && map->private_size < map->shrink_at)
+ rehash(map, map->tablesize >> HASHMAP_RESIZE_BITS);
+ }
+
return old;
}
diff --git a/hashmap.h b/hashmap.h
index 7a8fa7f..7b8e6f4 100644
--- a/hashmap.h
+++ b/hashmap.h
@@ -183,7 +183,7 @@ struct hashmap {
const void *cmpfn_data;
/* total number of entries (0 means the hashmap is empty) */
- unsigned int size;
+ unsigned int private_size; /* use hashmap_get_size() */
/*
* tablesize is the allocated size of the hash table. A non-0 value
@@ -196,8 +196,8 @@ struct hashmap {
unsigned int grow_at;
unsigned int shrink_at;
- /* See `hashmap_disallow_rehash`. */
- unsigned disallow_rehash : 1;
+ unsigned int do_count_items : 1;
+ unsigned int do_auto_rehash : 1;
};
/* hashmap functions */
@@ -253,6 +253,19 @@ static inline void hashmap_entry_init(void *entry,
unsigned int hash)
}
/*
+ * Return the number of items in the map.
+ */
+inline unsigned int hashmap_get_size(struct hashmap *map)
+{
+ if (map->do_count_items)
+ return map->private_size;
+
+ /* TODO Consider counting them and returning that. */
+ die("hashmap_get_size: size not set");
+ return 0;
+}
+
+/*
* Returns the hashmap entry for the specified key, or NULL if not found.
*
* `map` is the hashmap structure.
@@ -345,24 +358,6 @@ extern void *hashmap_remove(struct hashmap *map, const
void *key,
int hashmap_bucket(const struct hashmap *map, unsigned int hash);
/*
- * Disallow/allow rehashing of the hashmap.
- * This is useful if the caller knows that the hashmap needs multi-threaded
- * access. The caller is still required to guard/lock searches and inserts
- * in a manner appropriate to their usage. This simply prevents the table
- * from being unexpectedly re-mapped.
- *
- * It is up to the caller to ensure that the hashmap is initialized to a
- * reasonable size to prevent poor performance.
- *
- * A call to allow rehashing does not force a rehash; that might happen
- * with the next insert or delete.
- */
-static inline void hashmap_disallow_rehash(struct hashmap *map, unsigned value)
-{
- map->disallow_rehash = value;
-}
-
-/*
* Used to iterate over all entries of a hashmap. Note that it is
* not safe to add or remove entries to the hashmap while
* iterating.
@@ -387,6 +382,67 @@ static inline void *hashmap_iter_first(struct hashmap *map,
return hashmap_iter_next(iter);
}
+/*
+ * Disable item counting when adding/removing items.
+ *
+ * Normally, the hashmap keeps track of the number of items in the map
+ * and uses it to dynamically resize it. This (both the counting and
+ * the resizing) can cause problems when the map is being used by
+ * threaded callers (because the hashmap code does not know about the
+ * locking strategy used by the threaded callers and therefore, does
+ * not know how to protect the "private_size" counter).
+ *
+ * (This will implicitly disable auto resizing.)
+ */
+static inline void hashmap_disable_item_counting(struct hashmap *map)
+{
+ map->do_count_items = 0;
+}
+
+/*
+ * Re-enable item couting when adding/removing items.
+ * If counting is currently disabled, it will force count them.
+ *
+ * This might be used after leaving a threaded section.
+ */
+static inline void hashmap_enable_item_counting(struct hashmap *map)
+{
+ void *item;
+ unsigned int n = 0;
+ struct hashmap_iter iter;
+
+ hashmap_iter_init(map, &iter);
+ while ((item = hashmap_iter_next(&iter)))
+ n++;
+
+ map->do_count_items = 1;
+ map->private_size = n;
+}
+
+/*
+ * Disable automatic resizing when adding/removing items.
+ *
+ * This prevents the hashmap from resizing the table size and
+ * redistributing the items into new bucket chains. This causes
+ * problems when the map is being used by threaded callers (because
+ * the hashmap code does not know about the locking strategy used by
+ * the threaded callers and therefore, does not know how to safely
+ * build new chains while other threads may be walking them).
+ */
+static inline void hashmap_disable_auto_rehash(struct hashmap *map)
+{
+ map->do_auto_rehash = 0;
+}
+
+/*
+ * Re-enable automatic resizing when adding/removing items.
+ * This DOES NOT force an immediate rehash.
+ */
+static inline void hashmap_enable_auto_rehash(struct hashmap *map)
+{
+ map->do_auto_rehash = 1;
+}
+
/* String interning */
/*
diff --git a/name-hash.c b/name-hash.c
index 0e10f3e..829ff59 100644
--- a/name-hash.c
+++ b/name-hash.c
@@ -580,9 +580,11 @@ static void lazy_init_name_hash(struct index_state *istate)
NULL, istate->cache_nr);
if (lookup_lazy_params(istate)) {
- hashmap_disallow_rehash(&istate->dir_hash, 1);
+ hashmap_disable_item_counting(&istate->dir_hash);
+ hashmap_disable_auto_rehash(&istate->dir_hash);
threaded_lazy_init_name_hash(istate);
- hashmap_disallow_rehash(&istate->dir_hash, 0);
+ hashmap_enable_auto_rehash(&istate->dir_hash);
+ hashmap_enable_item_counting(&istate->dir_hash);
} else {
int nr;
for (nr = 0; nr < istate->cache_nr; nr++)
diff --git a/t/helper/test-hashmap.c b/t/helper/test-hashmap.c
index 095d739..84c57b9 100644
--- a/t/helper/test-hashmap.c
+++ b/t/helper/test-hashmap.c
@@ -237,7 +237,7 @@ int cmd_main(int argc, const char **argv)
} else if (!strcmp("size", cmd)) {
/* print table sizes */
- printf("%u %u\n", map.tablesize, map.size);
+ printf("%u %u\n", map.tablesize,
hashmap_get_size(&map));
} else if (!strcmp("intern", cmd) && l1) {
--
2.9.3
