[PATCH v2 08/11] grep: allow submodule functions to run in parallel
- From
Matheus Tavares <matheus.bernardino@usp.br>
- Date
- Sep 30, 2019, 01:50 UTC
- Message-ID
- <8070e154070e6eaef6f31077d15349e74147ff84.1569808052.git.matheus.bernardino@usp.br>
- In-Reply-To
- <cover.1569808052.git.matheus.bernardino@usp.br>
Now that object reading operations are internally protected, the submodule initialization functions at builtin/grep.c:grep_submodule() are very close to being thread-safe. Let's take a look at each call and remove from the critical section what we can, for better performance:
- submodule_from_path() and is_submodule_active() cannot be called in parallel yet only because they call repo_read_gitmodules() which contains, in its call stack, operations that would otherwise be in race condition with object reading (for example parse_object() and is_promisor_remote()). However, they only call repo_read_gitmodules() if it wasn't read before. So let's pre-read it before firing the threads and allow these two functions to safelly be called in parallel.
- repo_submodule_init() is already thread-safe, so remove it from the critical section without other necessary changes.
- The repo_read_gitmodules(&subrepo) call at grep_submodule() is safe as no other thread is performing object reading operations in the subrepo yet. However, threads might be working in the superproject, and this function calls add_to_alternates_memory() internally, which is racy with object readings in the superproject. So it must be kept protected for now. Let's add a "NEEDSWORK" to it, informing why it cannot be removed from the critical section yet.
- Finally, add_to_alternates_memory() must be kept protected by the same reason of the above item.
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br> --- builtin/grep.c | 38 ++++++++++++++++++++++---------------- 1 file changed, 22 insertions(+), 16 deletions(-)
diff --git a/builtin/grep.c b/builtin/grep.c index 1c4ff4a75f..c973ac46a7 100644 --- a/builtin/grep.c +++ b/builtin/grep.c @@ -401,25 +401,23 @@ static int grep_submodule(struct grep_opt *opt, struct grep_opt subopt; int hit; - /* - * NEEDSWORK: submodules functions need to be protected because they - * call config_from_gitmodules(): the latter contains in its call stack - * many thread-unsafe operations that are racy with object reading, such - * as parse_object() and is_promisor_object(). - */ - obj_read_lock(); sub = submodule_from_path(superproject, &null_oid, path); - if (!is_submodule_active(superproject, path)) { - obj_read_unlock(); + if (!is_submodule_active(superproject, path)) return 0; - } - if (repo_submodule_init(&subrepo, superproject, sub)) { - obj_read_unlock(); + if (repo_submodule_init(&subrepo, superproject, sub)) return 0; - } + /* + * NEEDSWORK: repo_read_gitmodules() might call + * add_to_alternates_memory() via config_from_gitmodules(). This + * operation causes a race condition with concurrent object readings + * performed by the worker threads. That's why we need obj_read_lock() + * here. It should be removed once it's no longer necessary to add the + * subrepo's odbs to the in-memory alternates list. + */ + obj_read_lock(); repo_read_gitmodules(&subrepo, 0); /* @@ -1052,6 +1050,9 @@ int cmd_grep(int argc, const char **argv, const char *prefix) pathspec.recursive = 1; pathspec.recurse_submodules = !!recurse_submodules; + if (recurse_submodules && (!use_index || untracked)) + die(_("option not supported with --recurse-submodules")); + if (list.nr || cached || show_in_pager) { if (num_threads > 1) warning(_("invalid option combination, ignoring --threads")); @@ -1071,6 +1072,14 @@ int cmd_grep(int argc, const char **argv, const char *prefix) && (opt.pre_context || opt.post_context || opt.file_break || opt.funcbody)) skip_first_line = 1; + + /* + * Pre-read gitmodules (if not read already) to prevent racy + * lazy reading in worker threads. + */ + if (recurse_submodules) + repo_read_gitmodules(the_repository, 1); + start_threads(&opt); } else { /* @@ -1105,9 +1114,6 @@ int cmd_grep(int argc, const char **argv, const char *prefix) } } - if (recurse_submodules && (!use_index || untracked)) - die(_("option not supported with --recurse-submodules")); - if (!show_in_pager && !opt.status_only) setup_pager();
-- 2.23.0