diff --git a/actionpack/lib/action_dispatch.rb b/actionpack/lib/action_dispatch.rb
index 01d49475deaa9a10c6c2796008e6d5744562ec31..17aa115f15e426249316b4ea50463809c90e6fde 100644
--- a/actionpack/lib/action_dispatch.rb
+++ b/actionpack/lib/action_dispatch.rb
@@ -50,6 +50,7 @@ class IllegalStateError < StandardError
     autoload :Callbacks
     autoload :Cookies
     autoload :DebugExceptions
+    autoload :DebugLocks
     autoload :ExceptionWrapper
     autoload :Executor
     autoload :Flash
diff --git a/actionpack/lib/action_dispatch/middleware/debug_locks.rb b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
new file mode 100644
index 0000000000000000000000000000000000000000..13254d08de38589f2462835b900fd2fc01a96800
--- /dev/null
+++ b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
@@ -0,0 +1,122 @@
+module ActionDispatch
+  # This middleware can be used to diagnose deadlocks in the autoload interlock.
+  #
+  # To use it, insert it near the top of the middleware stack, using
+  # <tt>config/application.rb</tt>:
+  #
+  #     config.middleware.insert_before Rack::Sendfile, ActionDispatch::DebugLocks
+  #
+  # After restarting the application and re-triggering the deadlock condition,
+  # <tt>/rails/locks</tt> will show a summary of all threads currently known to
+  # the interlock, which lock level they are holding or awaiting, and their
+  # current backtrace.
+  #
+  # Generally a deadlock will be caused by the interlock conflicting with some
+  # other external lock or blocking I/O call. These cannot be automatically
+  # identified, but should be visible in the displayed backtraces.
+  #
+  # NOTE: The formatting and content of this middleware's output is intended for
+  # human consumption, and should be expected to change between releases.
+  #
+  # This middleware exposes operational details of the server, with no access
+  # control. It should only be enabled when in use, and removed thereafter.
+  class DebugLocks
+    def initialize(app, path = '/rails/locks')
+      @app = app
+      @path = path
+    end
+
+    def call(env)
+      req = ActionDispatch::Request.new env
+
+      if req.get?
+        path = req.path_info.chomp('/'.freeze)
+        if path == @path
+          return render_details(req)
+        end
+      end
+
+      @app.call(env)
+    end
+
+    private
+      def render_details(req)
+        threads = ActiveSupport::Dependencies.interlock.raw_state do |threads|
+          # The Interlock itself comes to a complete halt as long as this block
+          # is executing. That gives us a more consistent picture of everything,
+          # but creates a pretty strong Observer Effect.
+          #
+          # Most directly, that means we need to do as little as possible in
+          # this block. More widely, it means this middleware should remain a
+          # strictly diagnostic tool (to be used when something has gone wrong),
+          # and not for any sort of general monitoring.
+
+          threads.each.with_index do |(thread, info), idx|
+            info[:index] = idx
+            info[:backtrace] = thread.backtrace
+          end
+
+          threads
+        end
+
+        str = threads.map do |thread, info|
+          if info[:exclusive]
+            lock_state = 'Exclusive'
+          elsif info[:sharing] > 0
+            lock_state = 'Sharing'
+            lock_state << " x#{info[:sharing]}" if info[:sharing] > 1
+          else
+            lock_state = 'No lock'
+          end
+
+          if info[:waiting]
+            lock_state << ' (yielded share)'
+          end
+
+          msg = "Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n"
+
+          if info[:sleeper]
+            msg << "  Waiting in #{info[:sleeper]}"
+            msg << " to #{info[:purpose].to_s.inspect}" unless info[:purpose].nil?
+            msg << "\n"
+
+            if info[:compatible]
+              compat = info[:compatible].map { |c| c == false ? "share" : c.to_s.inspect }
+              msg << "  may be pre-empted for: #{compat.join(', ')}\n"
+            end
+
+            blockers = threads.values.select { |binfo| blocked_by?(info, binfo, threads.values) }
+            msg << "  blocked by: #{blockers.map {|i| i[:index] }.join(', ')}\n" if blockers.any?
+          end
+
+          blockees = threads.values.select { |binfo| blocked_by?(binfo, info, threads.values) }
+          msg << "  blocking: #{blockees.map {|i| i[:index] }.join(', ')}\n" if blockees.any?
+
+          msg << "\n#{info[:backtrace].join("\n")}\n" if info[:backtrace]
+        end.join("\n\n---\n\n\n")
+
+        [200, { "Content-Type" => "text/plain", "Content-Length" => str.size }, [str]]
+      end
+
+      def blocked_by?(victim, blocker, all_threads)
+        return false if victim.equal?(blocker)
+
+        case victim[:sleeper]
+        when :start_sharing
+          blocker[:exclusive] ||
+            (!victim[:waiting] && blocker[:compatible] && !blocker[:compatible].include?(false))
+        when :start_exclusive
+          blocker[:sharing] > 0 ||
+            blocker[:exclusive] ||
+            (blocker[:compatible] && !blocker[:compatible].include?(victim[:purpose]))
+        when :yield_shares
+          blocker[:exclusive]
+        when :stop_exclusive
+          blocker[:exclusive] ||
+            victim[:compatible] &&
+            victim[:compatible].include?(blocker[:purpose]) &&
+            all_threads.all? { |other| !other[:compatible] || blocker.equal?(other) || other[:compatible].include?(blocker[:purpose]) }
+        end
+      end
+  end
+end
diff --git a/activesupport/lib/active_support/concurrency/share_lock.rb b/activesupport/lib/active_support/concurrency/share_lock.rb
index 89e63aefd4ff90dc9d8d784f1a8386552021a1ce..cf632ea7b0028e85ea9a5a90850a03c0e3b7ded5 100644
--- a/activesupport/lib/active_support/concurrency/share_lock.rb
+++ b/activesupport/lib/active_support/concurrency/share_lock.rb
@@ -14,6 +14,38 @@ class ShareLock
       # to upgrade share locks to exclusive.
 
 
+      def raw_state # :nodoc:
+        synchronize do
+          threads = @sleeping.keys | @sharing.keys | @waiting.keys
+          threads |= [@exclusive_thread] if @exclusive_thread
+
+          data = {}
+
+          threads.each do |thread|
+            purpose, compatible = @waiting[thread]
+
+            data[thread] = {
+              thread: thread,
+              sharing: @sharing[thread],
+              exclusive: @exclusive_thread == thread,
+              purpose: purpose,
+              compatible: compatible,
+              waiting: !!@waiting[thread],
+              sleeper: @sleeping[thread],
+            }
+          end
+
+          # NB: Yields while holding our *internal* synchronize lock,
+          # which is supposed to be used only for a few instructions at
+          # a time. This allows the caller to inspect additional state
+          # without things changing out from underneath, but would have
+          # disastrous effects upon normal operation. Fortunately, this
+          # method is only intended to be called when things have
+          # already gone wrong.
+          yield data
+        end
+      end
+
       def initialize
         super()
 
@@ -21,6 +53,7 @@ def initialize
 
         @sharing = Hash.new(0)
         @waiting = {}
+        @sleeping = {}
         @exclusive_thread = nil
         @exclusive_depth = 0
       end
@@ -46,7 +79,7 @@ def start_exclusive(purpose: nil, compatible: [], no_wait: false)
               return false if no_wait
 
               yield_shares(purpose: purpose, compatible: compatible, block_share: true) do
-                @cv.wait_while { busy_for_exclusive?(purpose) }
+                wait_for(:start_exclusive) { busy_for_exclusive?(purpose) }
               end
             end
             @exclusive_thread = Thread.current
@@ -69,7 +102,7 @@ def stop_exclusive(compatible: [])
 
             if eligible_waiters?(compatible)
               yield_shares(compatible: compatible, block_share: true) do
-                @cv.wait_while { @exclusive_thread || eligible_waiters?(compatible) }
+                wait_for(:stop_exclusive) { @exclusive_thread || eligible_waiters?(compatible) }
               end
             end
             @cv.broadcast
@@ -84,11 +117,11 @@ def start_sharing
           elsif @waiting[Thread.current]
             # We're nested inside a +yield_shares+ call: we'll resume as
             # soon as there isn't an exclusive lock in our way
-            @cv.wait_while { @exclusive_thread }
+            wait_for(:start_sharing) { @exclusive_thread }
           else
             # This is an initial / outermost share call: any outstanding
             # requests for an exclusive lock get to go first
-            @cv.wait_while { busy_for_sharing?(false) }
+            wait_for(:start_sharing) { busy_for_sharing?(false) }
           end
           @sharing[Thread.current] += 1
         end
@@ -153,7 +186,7 @@ def yield_shares(purpose: nil, compatible: [], block_share: false)
           yield
         ensure
           synchronize do
-            @cv.wait_while { @exclusive_thread && @exclusive_thread != Thread.current }
+            wait_for(:yield_shares) { @exclusive_thread && @exclusive_thread != Thread.current }
 
             if previous_wait
               @waiting[Thread.current] = previous_wait
@@ -181,6 +214,13 @@ def busy_for_sharing?(purpose)
       def eligible_waiters?(compatible)
         @waiting.any? { |t, (p, _)| compatible.include?(p) && @waiting.all? { |t2, (_, c2)| t == t2 || c2.include?(p) } }
       end
+
+      def wait_for(method)
+        @sleeping[Thread.current] = method
+        @cv.wait_while { yield }
+      ensure
+        @sleeping.delete Thread.current
+      end
     end
   end
 end
diff --git a/activesupport/lib/active_support/dependencies/interlock.rb b/activesupport/lib/active_support/dependencies/interlock.rb
index f1865ca2f892d0392d3435275ce88a4e02fb303f..75b769574c27544a2c05131e8123ede5ff59837c 100644
--- a/activesupport/lib/active_support/dependencies/interlock.rb
+++ b/activesupport/lib/active_support/dependencies/interlock.rb
@@ -46,6 +46,10 @@ def permit_concurrent_loads
           yield
         end
       end
+
+      def raw_state(&block) # :nodoc:
+        @lock.raw_state(&block)
+      end
     end
   end
 end