diff --git a/lib/sshkit/backends/netssh.rb b/lib/sshkit/backends/netssh.rb
index 7211c7c9..03be64cb 100644
--- a/lib/sshkit/backends/netssh.rb
+++ b/lib/sshkit/backends/netssh.rb
@@ -1,6 +1,5 @@
 require 'English'
 require 'strscan'
-require 'mutex_m'
 require 'net/ssh'
 
 module Net
diff --git a/lib/sshkit/backends/netssh/known_hosts.rb b/lib/sshkit/backends/netssh/known_hosts.rb
index 869d406a..bb4ee8ac 100644
--- a/lib/sshkit/backends/netssh/known_hosts.rb
+++ b/lib/sshkit/backends/netssh/known_hosts.rb
@@ -7,12 +7,11 @@ module Backend
     class Netssh < Abstract
 
       class KnownHostsKeys
-        include Mutex_m
-
         def initialize(path)
           super()
           @path = File.expand_path(path)
           @hosts_keys = nil
+          @mutex = Mutex.new
         end
 
         def keys_for(hostlist)
@@ -46,7 +45,7 @@ def sha1
         end
 
         def parse_file
-          synchronize do
+          @mutex.synchronize do
             return if hosts_keys && hosts_hashes
 
             unless File.readable?(path)
@@ -112,11 +111,10 @@ def parse_key(scanner)
       end
 
       class KnownHosts
-        include Mutex_m
-
         def initialize
           super()
           @files = {}
+          @mutex = Mutex.new
         end
 
         def search_for(host, options = {})
@@ -128,13 +126,13 @@ def search_for(host, options = {})
 
         def add(*args)
           ::Net::SSH::KnownHosts.add(*args)
-          synchronize { @files = {} }
+          @mutex.synchronize { @files = {} }
         end
 
         private
 
         def known_hosts_file(path)
-          @files[path] || synchronize { @files[path] ||= KnownHostsKeys.new(path) }
+          @files[path] || @mutex.synchronize { @files[path] ||= KnownHostsKeys.new(path) }
         end
       end
 
diff --git a/sshkit.gemspec b/sshkit.gemspec
index 1584bcfe..416ccb18 100644
--- a/sshkit.gemspec
+++ b/sshkit.gemspec
@@ -21,7 +21,6 @@ Gem::Specification.new do |gem|
   gem.version       = SSHKit::VERSION
 
   gem.add_runtime_dependency('base64') if RUBY_VERSION >= "2.4"
-  gem.add_runtime_dependency('mutex_m')
   gem.add_runtime_dependency('net-ssh',  '>= 2.8.0')
   gem.add_runtime_dependency('net-scp',  '>= 1.1.2')
   gem.add_runtime_dependency('net-sftp', '>= 2.1.2')