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 safely 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 for the
same reason as the item above.
Signed-off-by: Matheus Tavares <matheus.bernardino@xxxxxx>
---
builtin/grep.c | 38 ++++++++++++++++++++++----------------
1 file changed, 22 insertions(+), 16 deletions(-)
diff --git a/builtin/grep.c b/builtin/grep.c
index d3ed05c1da..ac3d86c2e5 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();
--
2.24.1
