diff --git a/.github/workflows/auto_review_pr.yml b/.github/workflows/auto_review_pr.yml
new file mode 100644
index 00000000000000..c8095dfd8e34a0
--- /dev/null
+++ b/.github/workflows/auto_review_pr.yml
@@ -0,0 +1,33 @@
+name: Auto Review PR
+on:
+  pull_request_target:
+    types: [opened, ready_for_review, reopened]
+    branches: [master]
+
+permissions:
+  contents: read
+
+jobs:
+  auto-review-pr:
+    name: Auto Review PR
+    runs-on: ubuntu-latest
+    if: ${{ github.repository == 'ruby/ruby' && github.base_ref == 'master' }}
+
+    permissions:
+      pull-requests: write
+      contents: read
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - uses: ruby/setup-ruby@ab177d40ee5483edb974554986f56b33477e21d0 # v1.265.0
+        with:
+          ruby-version: '3.4'
+          bundler: none
+
+      - name: Auto Review PR
+        run: ruby tool/auto_review_pr.rb "$GITHUB_PR_NUMBER"
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_PR_NUMBER: ${{ github.event.pull_request.number }}
diff --git a/.github/workflows/sync_default_gems.yml b/.github/workflows/sync_default_gems.yml
index 2f0b4d8865eb4f..1a9b43ec5241a9 100644
--- a/.github/workflows/sync_default_gems.yml
+++ b/.github/workflows/sync_default_gems.yml
@@ -56,7 +56,7 @@ jobs:
 
       - name: Push
         run: |
-          git pull --ff-only origin ${GITHUB_REF#refs/heads/}
+          git pull --rebase origin ${GITHUB_REF#refs/heads/}
           git push origin ${GITHUB_REF#refs/heads/}
         if: ${{ steps.sync.outputs.update }}
 
diff --git a/common.mk b/common.mk
index fe8ef3b19efe43..093fea6b3fb56c 100644
--- a/common.mk
+++ b/common.mk
@@ -624,15 +624,20 @@ html: PHONY main srcs-doc
 	@echo Generating RDoc HTML files
 	$(Q) $(RDOC) --op "$(HTMLOUT)" $(RDOC_GEN_OPTS) $(RDOCFLAGS) .
 
+RDOC_COVERAGE_EXCLUDES = -x ^ext/json -x ^ext/openssl -x ^ext/psych \
+	-x ^lib/bundler -x ^lib/rubygems \
+	-x ^lib/did_you_mean -x ^lib/error_highlight -x ^lib/syntax_suggest
+
 rdoc-coverage: PHONY main srcs-doc
 	@echo Generating RDoc coverage report
-	$(Q) $(RDOC) --quiet -C $(RDOCFLAGS) .
+	$(Q) $(RDOC) --quiet -C $(RDOCFLAGS) $(RDOC_COVERAGE_EXCLUDES) .
 
 undocumented: PHONY main srcs-doc
-	$(Q) $(RDOC) --quiet -C $(RDOCFLAGS) . | \
+	$(Q) $(RDOC) --quiet -C $(RDOCFLAGS) $(RDOC_COVERAGE_EXCLUDES) . | \
 	sed -n \
 	-e '/^ *# in file /{' -e 's///;N;s/\n/: /p' -e '}' \
-	-e 's/^ *\(.*[^ ]\) *# in file \(.*\)/\2: \1/p' | sort
+	-e 's/^ *\(.*[^ ]\) *# in file \(.*\)/\2: \1/p' | \
+	sort -t: -k1,1 -k2n,2
 
 RDOCBENCHOUT=/tmp/rdocbench
 
diff --git a/ext/coverage/lib/coverage.rb b/ext/coverage/lib/coverage.rb
index f1923ef366cbf3..4bd20e22cbadbe 100644
--- a/ext/coverage/lib/coverage.rb
+++ b/ext/coverage/lib/coverage.rb
@@ -1,6 +1,11 @@
 require "coverage.so"
 
 module Coverage
+  # call-seq:
+  #   line_stub(file) -> array
+  #
+  # A simple helper function that creates the "stub" of line coverage
+  # from a given source code.
   def self.line_stub(file)
     lines = File.foreach(file).map { nil }
     iseqs = [RubyVM::InstructionSequence.compile_file(file)]
diff --git a/ext/objspace/object_tracing.c b/ext/objspace/object_tracing.c
index 0156642ef25897..63eec6739320f9 100644
--- a/ext/objspace/object_tracing.c
+++ b/ext/objspace/object_tracing.c
@@ -411,6 +411,13 @@ object_allocations_reporter(FILE *out, void *ptr)
     fprintf(out, "== object_allocations_reporter: END\n");
 }
 
+/*
+ * call-seq: trace_object_allocations_debug_start
+ *
+ * Starts tracing object allocations for GC debugging.
+ * If you encounter the BUG "... is T_NONE" (and so on) on your
+ * application, please try this method at the beginning of your app.
+ */
 static VALUE
 trace_object_allocations_debug_start(VALUE self)
 {
diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index 9dd3129e01664e..cc0b1afd4d2051 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -1022,16 +1022,20 @@ rb_gc_impl_shutdown_call_finalizer(void *objspace_ptr)
         gc_run_finalizers(objspace);
     }
 
-    struct MMTk_RawVecOfObjRef registered_candidates = mmtk_get_all_obj_free_candidates();
-    for (size_t i = 0; i < registered_candidates.len; i++) {
-        VALUE obj = (VALUE)registered_candidates.ptr[i];
-
-        if (rb_gc_shutdown_call_finalizer_p(obj)) {
-            rb_gc_obj_free(objspace_ptr, obj);
-            RBASIC(obj)->flags = 0;
+    unsigned int lev = RB_GC_VM_LOCK();
+    {
+        struct MMTk_RawVecOfObjRef registered_candidates = mmtk_get_all_obj_free_candidates();
+        for (size_t i = 0; i < registered_candidates.len; i++) {
+            VALUE obj = (VALUE)registered_candidates.ptr[i];
+
+            if (rb_gc_shutdown_call_finalizer_p(obj)) {
+                rb_gc_obj_free(objspace_ptr, obj);
+                RBASIC(obj)->flags = 0;
+            }
         }
+        mmtk_free_raw_vec_of_obj_ref(registered_candidates);
     }
-    mmtk_free_raw_vec_of_obj_ref(registered_candidates);
+    RB_GC_VM_UNLOCK(lev);
 
     gc_run_finalizers(objspace);
 }
diff --git a/lib/net/http.rb b/lib/net/http.rb
index 43e3349ac0e1b1..7efb468e782e06 100644
--- a/lib/net/http.rb
+++ b/lib/net/http.rb
@@ -1825,6 +1825,8 @@ def HTTP.Proxy(p_addr = :ENV, p_port = nil, p_user = nil, p_pass = nil, p_use_ss
       }
     end
 
+    # :startdoc:
+
     class << HTTP
       # Returns true if self is a class which was created by HTTP::Proxy.
       def proxy_class?
@@ -1948,6 +1950,7 @@ def edit_path(path)
         path
       end
     end
+    # :startdoc:
 
     #
     # HTTP operations
diff --git a/tool/auto_review_pr.rb b/tool/auto_review_pr.rb
new file mode 100755
index 00000000000000..c63640354d3593
--- /dev/null
+++ b/tool/auto_review_pr.rb
@@ -0,0 +1,104 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require 'json'
+require 'net/http'
+require 'uri'
+require_relative './sync_default_gems'
+
+class GitHubAPIClient
+  def initialize(token)
+    @token = token
+  end
+
+  def get(path)
+    response = Net::HTTP.get_response(URI("https://api.github.com#{path}"), {
+      'Authorization' => "token #{@token}",
+      'Accept' => 'application/vnd.github.v3+json',
+    }).tap(&:value)
+    JSON.parse(response.body, symbolize_names: true)
+  end
+
+  def post(path, body = {})
+    body = JSON.dump(body)
+    response = Net::HTTP.post(URI("https://api.github.com#{path}"), body, {
+      'Authorization' => "token #{@token}",
+      'Accept' => 'application/vnd.github.v3+json',
+      'Content-Type' => 'application/json',
+    }).tap(&:value)
+    JSON.parse(response.body, symbolize_names: true)
+  end
+end
+
+class AutoReviewPR
+  REPO = 'ruby/ruby'
+
+  COMMENT_USER = 'github-actions[bot]'
+  COMMENT_PREFIX = 'The following files are maintained in the following upstream repositories:'
+  COMMENT_SUFFIX = 'Please file a pull request to the above instead. Thank you!'
+
+  def initialize(client)
+    @client = client
+  end
+
+  def review(pr_number)
+    # Fetch the list of files changed by the PR
+    changed_files = @client.get("/repos/#{REPO}/pulls/#{pr_number}/files").map { it.fetch(:filename) }
+
+    # Build a Hash: { upstream_repo => files, ... }
+    upstream_repos = changed_files.group_by { |file| find_upstream_repo(file) }
+    upstream_repos.delete(nil) # exclude no-upstream files
+    upstream_repos.delete('prism') if changed_files.include?('prism_compile.c') # allow prism changes in this case
+    if upstream_repos.empty?
+      puts "Skipped: The PR ##{pr_number} doesn't have upstream repositories."
+      return
+    end
+
+    # Check if the PR is already reviewed
+    existing_comments = @client.get("/repos/#{REPO}/issues/#{pr_number}/comments")
+    existing_comments.map! { [it.fetch(:user).fetch(:login), it.fetch(:body)] }
+    if existing_comments.any? { |user, comment| user == COMMENT_USER && comment.start_with?(COMMENT_PREFIX) }
+      puts "Skipped: The PR ##{pr_number} already has an automated review comment."
+      return
+    end
+
+    # Post a comment
+    comment = format_comment(upstream_repos)
+    result = @client.post("/repos/#{REPO}/issues/#{pr_number}/comments", { body: comment })
+    puts "Success: #{JSON.pretty_generate(result)}"
+  end
+
+  private
+
+  def find_upstream_repo(file)
+    SyncDefaultGems::REPOSITORIES.each do |repo_name, repository|
+      repository.mappings.each do |_src, dst|
+        if file.start_with?(dst)
+          return repo_name
+        end
+      end
+    end
+    nil
+  end
+
+  # upstream_repos: { upstream_repo => files, ... }
+  def format_comment(upstream_repos)
+    comment = +''
+    comment << "#{COMMENT_PREFIX}\n\n"
+
+    upstream_repos.each do |upstream_repo, files|
+      comment << "* https://github.com/ruby/#{upstream_repo}\n"
+      files.each do |file|
+        comment << "    * #{file}\n"
+      end
+    end
+
+    comment << "\n#{COMMENT_SUFFIX}"
+    comment
+  end
+end
+
+pr_number = ARGV[0] || abort("Usage: #{$0} <pr_number>")
+client = GitHubAPIClient.new(ENV.fetch('GITHUB_TOKEN'))
+
+AutoReviewPR.new(client).review(pr_number)
diff --git a/tool/sync_default_gems.rb b/tool/sync_default_gems.rb
index b2509f0062b0b5..997cd5eddc8f16 100755
--- a/tool/sync_default_gems.rb
+++ b/tool/sync_default_gems.rb
@@ -61,6 +61,7 @@ def lib((upstream, branch), gemspec_in_subdir: false)
     ])
   end
 
+  # Note: tool/auto_review_pr.rb also depends on this constant.
   REPOSITORIES = {
     "io-console": repo("ruby/io-console", [
       ["ext/io/console", "ext/io/console"],