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authorJeff Hostetler <jeffhost@microsoft.com>2017-09-06 15:43:48 (GMT)
committerJunio C Hamano <gitster@pobox.com>2017-09-07 00:42:02 (GMT)
commit8b604d19515c4be18403047045faa363d4de217b (patch)
tree5bb6fb5e265c3f46c0a08b498c6ee2159dc326cc /name-hash.c
parent238e487ea943f80734cc6dad665e7238b8cbc7ff (diff)
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hashmap: add API to disable item counting when threaded
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.agren@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=oyxe4EXXOmCKvsG5MSuzxsA@mail.gmail.com/ Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'name-hash.c')
-rw-r--r--name-hash.c10
1 files changed, 8 insertions, 2 deletions
diff --git a/name-hash.c b/name-hash.c
index bd8dc7a..45c98db 100644
--- a/name-hash.c
+++ b/name-hash.c
@@ -584,9 +584,15 @@ static void lazy_init_name_hash(struct index_state *istate)
hashmap_init(&istate->dir_hash, dir_entry_cmp, 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++)