[PATCH v3 04/12] replace-object: make replace operations thread-safe
- From
Matheus Tavares <matheus.bernardino@usp.br>
- Date
- Jan 16, 2020, 02:39 UTC
- Message-ID
- <d7f739bc57b6f59cab7c718300c28b8c6b0a61a8.1579141989.git.matheus.bernardino@usp.br>
- In-Reply-To
- <cover.1579141989.git.matheus.bernardino@usp.br>
replace-object functions are very close to being thread-safe: the only current racy section is the lazy initialization at prepare_replace_object(). The following patches will protect some object reading operations to be called threaded, but before that, replace functions must be protected. To do so, add a mutex to struct raw_object_store and acquire it before lazy initializing the replace_map. This won't cause any noticeable performance drop as the mutex will no longer be used after the replace_map is initialized.
Later, when the replace functions are called in parallel, thread debuggers might point our use of the added replace_map_initialized flag as a data race. However, as this boolean variable is initialized as false and it's only updated once, there's no real harm. It's perfectly fine if the value is updated right after a thread read it in replace-map.h:lookup_replace_object() (there'll only be a performance penalty for the affected threads at that moment). We could cease the debugger warning protecting the variable reading at the said function. However, this would negatively affect performance for all threads calling it, at any time, so it's not really worthy since the warning doesn't represent a real problem. Instead, to make sure we don't get false positives (at ThreadSanitizer, at least) an entry for the respective function is added to .tsan-suppressions.
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br> --- .tsan-suppressions | 6 ++++++ object-store.h | 2 ++ object.c | 2 ++ replace-object.c | 11 ++++++++++- replace-object.h | 7 ++++++- 5 files changed, 26 insertions(+), 2 deletions(-)
diff --git a/.tsan-suppressions b/.tsan-suppressions index 8c85014a0a..5ba86d6845 100644 --- a/.tsan-suppressions +++ b/.tsan-suppressions @@ -8,3 +8,9 @@ # in practice it (hopefully!) doesn't matter. race:^want_color$ race:^transfer_debug$ + +# A boolean value, which tells whether the replace_map has been initialized or +# not, is read racily with an update. As this variable is written to only once, +# and it's OK if the value change right after reading it, this shouldn't be a +# problem. +race:^lookup_replace_object$ diff --git a/object-store.h b/object-store.h index 55ee639350..33739c9dee 100644 --- a/object-store.h +++ b/object-store.h @@ -125,6 +125,8 @@ struct raw_object_store { * (see git-replace(1)). */ struct oidmap *replace_map; + unsigned replace_map_initialized : 1; + pthread_mutex_t replace_mutex; /* protect object replace functions */ struct commit_graph *commit_graph; unsigned commit_graph_attempted : 1; /* if loading has been attempted */ diff --git a/object.c b/object.c index 142ef69399..b4e1d3db3c 100644 --- a/object.c +++ b/object.c @@ -480,6 +480,7 @@ struct raw_object_store *raw_object_store_new(void) memset(o, 0, sizeof(*o)); INIT_LIST_HEAD(&o->packed_git_mru); hashmap_init(&o->pack_map, pack_map_entry_cmp, NULL, 0); + pthread_mutex_init(&o->replace_mutex, NULL); return o; } @@ -507,6 +508,7 @@ void raw_object_store_clear(struct raw_object_store *o) oidmap_free(o->replace_map, 1); FREE_AND_NULL(o->replace_map); + pthread_mutex_destroy(&o->replace_mutex); free_commit_graph(o->commit_graph); o->commit_graph = NULL; diff --git a/replace-object.c b/replace-object.c index e295e87943..7bd9aba6ee 100644 --- a/replace-object.c +++ b/replace-object.c @@ -34,14 +34,23 @@ static int register_replace_ref(struct repository *r, void prepare_replace_object(struct repository *r) { - if (r->objects->replace_map) + if (r->objects->replace_map_initialized) return; + pthread_mutex_lock(&r->objects->replace_mutex); + if (r->objects->replace_map_initialized) { + pthread_mutex_unlock(&r->objects->replace_mutex); + return; + } + r->objects->replace_map = xmalloc(sizeof(*r->objects->replace_map)); oidmap_init(r->objects->replace_map, 0); for_each_replace_ref(r, register_replace_ref, NULL); + r->objects->replace_map_initialized = 1; + + pthread_mutex_unlock(&r->objects->replace_mutex); } /* We allow "recursive" replacement. Only within reason, though */ diff --git a/replace-object.h b/replace-object.h index 04ed7a85a2..3fbc32eb7b 100644 --- a/replace-object.h +++ b/replace-object.h @@ -24,12 +24,17 @@ const struct object_id *do_lookup_replace_object(struct repository *r, * name (replaced recursively, if necessary). The return value is * either sha1 or a pointer to a permanently-allocated value. When * object replacement is suppressed, always return sha1. + * + * Note: some thread debuggers might point a data race on the + * replace_map_initialized reading in this function. However, we know there's no + * problem in the value being updated by one thread right after another one read + * it here (and it should be written to only once, anyway). */ static inline const struct object_id *lookup_replace_object(struct repository *r, const struct object_id *oid) { if (!read_replace_refs || - (r->objects->replace_map && + (r->objects->replace_map_initialized && r->objects->replace_map->map.tablesize == 0)) return oid; return do_lookup_replace_object(r, oid);
-- 2.24.1