Prevent starvation when acquiring mutex over and over (#15877)

Continually locking a mutex m can lead to starvation if all other threads are on the waitq of m.

See https://bugs.ruby-lang.org/issues/21840 for more details.

Solution:

When a thread `T1` wakes up `T2` during mutex unlock but `T1` or any other thread successfully acquires it
before `T2`, then we record the `running_time` of the thread during mutex acquisition. Then during unlock, if
that thread's running_time is less than the saved running time, we set it back to the saved time.

Fixes [Bug #21840]

test/ruby/test_thread.rb

@@ -1664,4 +1664,37 @@ q.pop
       assert_operator elapsed, :>=, 0.1, "sub-millisecond sleeps should not return immediately"
     end;
   end
+
+  # [Bug #21840]
+  def test_mutex_owner_doesnt_starve_waiters
+    assert_ruby_status([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      m = Mutex.new
+
+      fib = lambda { |n|
+        return n if n <= 1
+        fib(n - 1) + fib(n - 2)
+      }
+
+      t1_running = false
+      t1 = Thread.new do
+        t1_running = true
+        loop do
+          fib(20)
+          m.synchronize do
+            File.open(__FILE__) { } # reset timeslice due to blocking operation
+          end
+        end
+      end
+
+      loop until t1_running
+
+      3.times.map do
+        Thread.new do
+          m.synchronize do
+          end
+        end
+      end.each(&:join)
+    end;
+  end
 end

thread_sync.c

@@ -10,6 +10,8 @@ typedef struct rb_mutex_struct {
     rb_thread_t *th; // even if the fiber is collected, we might need access to the thread in mutex_free
     struct rb_mutex_struct *next_mutex;
     struct ccan_list_head waitq; /* protected by GVL */
+    uint32_t saved_running_time_us;
+    bool wait_waking; // Is there a thread waiting to be woken up by this mutex? Reset during every wakeup.
 } rb_mutex_t;
 
 /* sync_waiter is always on-stack */
@@ -212,8 +214,15 @@ mutex_locked(rb_mutex_t *mutex, rb_thread_t *th, rb_serial_t ec_serial)
 static inline bool
 do_mutex_trylock(rb_mutex_t *mutex, rb_thread_t *th, rb_serial_t ec_serial)
 {
+    // NOTE: we can successfully lock a mutex even if there are other threads waiting on it. First one to it wins.
     if (mutex->ec_serial == 0) {
         RUBY_DEBUG_LOG("%p ok", mutex);
+        if (mutex->wait_waking) {
+            // If we acquired `mutex` without contention and before the thread that was popped off the waitq, we're going
+            // to set our running_time back to what it was here during mutex unlock if it got reset during our critical
+            // section. This is to prevent starvation of other threads waiting on the mutex.
+            mutex->saved_running_time_us = th->running_time_us;
+        }
 
         mutex_locked(mutex, th, ec_serial);
         return true;
@@ -350,7 +359,8 @@ do_mutex_lock(struct mutex_args *args, int interruptible_p)
                 }
                 ccan_list_del(&sync_waiter.node);
 
-                // unlocked by another thread while sleeping
+                // If mutex->ec_serial != 0, the mutex was locked by another thread before we had the chance to acquire it.
+                // We'll put ourselves on the waitq and sleep again.
                 if (!mutex->ec_serial) {
                     mutex_set_owner(mutex, th, ec_serial);
                 }
@@ -391,6 +401,7 @@ do_mutex_lock(struct mutex_args *args, int interruptible_p)
 
         if (saved_ints) th->ec->interrupt_flag = saved_ints;
         if (mutex->ec_serial == ec_serial) mutex_locked(mutex, th, ec_serial);
+        mutex->wait_waking = false;
     }
 
     RUBY_DEBUG_LOG("%p locked", mutex);
@@ -454,6 +465,15 @@ rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th, rb_serial_t ec_serial)
 
     struct sync_waiter *cur = 0, *next;
 
+
+    if (mutex->wait_waking) {
+        uint32_t saved = mutex->saved_running_time_us;
+        if (th->running_time_us < saved) {
+            th->running_time_us = saved;
+        }
+    }
+
+    mutex->saved_running_time_us = 0;
     mutex->ec_serial = 0;
     thread_mutex_remove(th, mutex);
 
@@ -469,6 +489,7 @@ rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th, rb_serial_t ec_serial)
               case THREAD_RUNNABLE: /* from someone else calling Thread#run */
               case THREAD_STOPPED_FOREVER: /* likely (rb_mutex_lock) */
                 RUBY_DEBUG_LOG("wakeup th:%u", rb_th_serial(cur->th));
+                mutex->wait_waking = true;
                 rb_threadptr_interrupt(cur->th);
                 return NULL;
               case THREAD_STOPPED: /* probably impossible */
@@ -480,7 +501,6 @@ rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th, rb_serial_t ec_serial)
             }
         }
     }
-
     // We did not find any threads to wake up, so we can just return with no error:
     return NULL;
 }