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 thus automatic rehashing. Also include API to later re-enable them. 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 this. Here is the relevant output from ThreadSanitizer showing the problem: WARNING: ThreadSanitizer: data race (pid=10554) Read of size 4 at 0x00000082d488 by thread T2 (mutexes: write M16): #0 hashmap_add hashmap.c:209 #1 hash_dir_entry_with_parent_and_prefix name-hash.c:302 #2 handle_range_dir name-hash.c:347 #3 handle_range_1 name-hash.c:415 #4 lazy_dir_thread_proc name-hash.c:471 #5 <null> <null> Previous write of size 4 at 0x00000082d488 by thread T1 (mutexes: write M31): #0 hashmap_add hashmap.c:209 #1 hash_dir_entry_with_parent_and_prefix name-hash.c:302 #2 handle_range_dir name-hash.c:347 #3 handle_range_1 name-hash.c:415 #4 handle_range_dir name-hash.c:380 #5 handle_range_1 name-hash.c:415 #6 lazy_dir_thread_proc name-hash.c:471 #7 <null> <null> Martin gives instructions for running TSan on test t3008 in this post: https://public-inbox.org/git/CAN0heSoJDL9pWELD6ciLTmWf-a=oyxe4exxomckvsg5msuz...@mail.gmail.com/ Signed-off-by: Jeff Hostetler <jeffh...@microsoft.com> --- attr.c | 15 ++++++----- builtin/describe.c | 2 +- hashmap.c | 26 +++++++++++------- hashmap.h | 72 ++++++++++++++++++++++++++++++++++--------------- name-hash.c | 10 +++++-- t/helper/test-hashmap.c | 3 ++- 6 files changed, 88 insertions(+), 40 deletions(-) diff --git a/attr.c b/attr.c index 56961f0..195aa6d 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 +237,11 @@ 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..d42f01f 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 +163,12 @@ void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function, while (initial_size > size) size <<= HASHMAP_RESIZE_BITS; alloc_table(map, size); + + /* + * Keep track of the number of items in the map and + * allow the map to automatically grow as necessary. + */ + map->do_count_items = 1; } void hashmap_free(struct hashmap *map, int free_entries) @@ -206,9 +209,11 @@ 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->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 +229,12 @@ 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->private_size < map->shrink_at) + rehash(map, map->tablesize >> HASHMAP_RESIZE_BITS); + } + return old; } diff --git a/hashmap.h b/hashmap.h index 7a8fa7f..7cb29a6 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 +196,7 @@ struct hashmap { unsigned int grow_at; unsigned int shrink_at; - /* See `hashmap_disallow_rehash`. */ - unsigned disallow_rehash : 1; + unsigned int do_count_items : 1; }; /* hashmap functions */ @@ -253,6 +252,18 @@ static inline void hashmap_entry_init(void *entry, unsigned int hash) } /* + * Return the number of items in the map. + */ +static inline unsigned int hashmap_get_size(struct hashmap *map) +{ + if (map->do_count_items) + return map->private_size; + + BUG("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 +356,6 @@ 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 +380,43 @@ static inline void *hashmap_iter_first(struct hashmap *map, return hashmap_iter_next(iter); } +/* + * Disable item counting and automatic rehashing 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). + */ +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. + * It WILL NOT automatically rehash them. + */ +static inline void hashmap_enable_item_counting(struct hashmap *map) +{ + void *item; + unsigned int n = 0; + struct hashmap_iter iter; + + if (map->do_count_items) + return; + + hashmap_iter_init(map, &iter); + while ((item = hashmap_iter_next(&iter))) + n++; + + map->do_count_items = 1; + map->private_size = n; +} + /* String interning */ /* diff --git a/name-hash.c b/name-hash.c index 0e10f3e..3ac2e57 100644 --- a/name-hash.c +++ b/name-hash.c @@ -580,9 +580,15 @@ 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); + /* + * Disable item counting and automatic rehashing because + * we do per-chain (mod n) locking rather than whole hashmap + * locking and we need to prevent the table-size from changing + * and bucket items from being redistributed. + */ + hashmap_disable_item_counting(&istate->dir_hash); threaded_lazy_init_name_hash(istate); - hashmap_disallow_rehash(&istate->dir_hash, 0); + 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..4956d1d 100644 --- a/t/helper/test-hashmap.c +++ b/t/helper/test-hashmap.c @@ -237,7 +237,8 @@ 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
