diff --git a/lib/rubygems.rb b/lib/rubygems.rb
index 80708e2aa0..37984157de 100644
--- a/lib/rubygems.rb
+++ b/lib/rubygems.rb
@@ -800,11 +800,11 @@ An Array (#{env.inspect}) was passed in from #{caller[3]}
   ##
   # Safely write a file in binary mode on all platforms.
   def self.write_binary(path, data)
+    File.open(path, File::RDWR | File::CREAT | File::BINARY | File::LOCK_EX) do |io|
+      io.write data
+    end
+  rescue *WRITE_BINARY_ERRORS
     File.open(path, 'wb') do |io|
-      begin
-        io.flock(File::LOCK_EX)
-      rescue *WRITE_BINARY_ERRORS
-      end
       io.write data
     end
   rescue Errno::ENOLCK # NFS
diff --git a/lib/rubygems/installer.rb b/lib/rubygems/installer.rb
index 38642ee8ef..8e3965ef92 100644
--- a/lib/rubygems/installer.rb
+++ b/lib/rubygems/installer.rb
@@ -446,13 +446,9 @@ class Gem::Installer
   # specifications directory.
 
   def write_spec
-    File.open spec_file, 'w' do |file|
-      spec.installed_by_version = Gem.rubygems_version
+    spec.installed_by_version = Gem.rubygems_version
 
-      file.puts spec.to_ruby_for_cache
-
-      file.fsync rescue nil # for filesystems without fsync(2)
-    end
+    Gem.write_binary(spec_file, spec.to_ruby_for_cache)
   end
 
   ##
@@ -460,9 +456,7 @@ class Gem::Installer
   # specifications/default directory.
 
   def write_default_spec
-    File.open(default_spec_file, "w") do |file|
-      file.puts spec.to_ruby
-    end
+    Gem.write_binary(default_spec_file, spec.to_ruby)
   end
 
   ##
diff --git a/test/rubygems/test_gem_installer.rb b/test/rubygems/test_gem_installer.rb
index 8874577aa8..dae2b070d5 100644
--- a/test/rubygems/test_gem_installer.rb
+++ b/test/rubygems/test_gem_installer.rb
@@ -288,6 +288,33 @@ gem 'other', version
                  "(SyntaxError)", e.message
   end
 
+  def test_ensure_no_race_conditions_between_installing_and_loading_gemspecs
+    a, a_gem = util_gem 'a', 2
+
+    Gem::Installer.at(a_gem).install
+
+    t1 = Thread.new do
+      5.times do
+        Gem::Installer.at(a_gem).install
+        sleep 0.1
+      end
+    end
+
+    t2 = Thread.new do
+      _, err = capture_output do
+        20.times do
+          Gem::Specification.load(a.spec_file)
+          Gem::Specification.send(:clear_load_cache)
+        end
+      end
+
+      assert_empty err
+    end
+
+    t1.join
+    t2.join
+  end
+
   def test_ensure_loadable_spec_security_policy
     pend 'openssl is missing' unless Gem::HAVE_OPENSSL