From d2f11d48b29de49265f1a0d997548074f80fe431 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gustav=20S=C3=B6rn=C3=A4s?= <gustav@sornas.net>
Date: Mon, 15 Mar 2021 13:50:45 +0100
Subject: add rwlock implementation

---
 src/threads/synch.c | 40 ++++++++++++++++++++++++++++++++++++++++
 src/threads/synch.h | 17 +++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/src/threads/synch.c b/src/threads/synch.c
index 317c68a..e714ce0 100644
--- a/src/threads/synch.c
+++ b/src/threads/synch.c
@@ -336,3 +336,43 @@ cond_broadcast (struct condition *cond, struct lock *lock)
   while (!list_empty (&cond->waiters))
     cond_signal (cond, lock);
 }
+
+void
+rwlock_init (struct rwlock *rwlock)
+{
+  sema_init (&rwlock->resource, 1);
+  lock_init (&rwlock->rmutex);
+  rwlock->readcount = 0;
+}
+
+void
+rwlock_write_p (struct rwlock *rwlock)
+{
+  sema_down (&rwlock->resource);
+}
+
+void
+rwlock_write_v (struct rwlock *rwlock)
+{
+  sema_up (&rwlock->resource);
+}
+
+void
+rwlock_read_p (struct rwlock *rwlock)
+{
+  lock_acquire (&rwlock->rmutex);
+  rwlock->readcount ++;
+  if (rwlock->readcount == 1)
+    sema_down (&rwlock->resource);
+  lock_release (&rwlock->rmutex);
+}
+
+void
+rwlock_read_v (struct rwlock *rwlock)
+{
+  lock_acquire (&rwlock->rmutex);
+  rwlock->readcount --;
+  if (rwlock->readcount == 0)
+    sema_up (&rwlock->resource);
+  lock_release (&rwlock->rmutex);
+}
diff --git a/src/threads/synch.h b/src/threads/synch.h
index a19e88b..a76eecc 100644
--- a/src/threads/synch.h
+++ b/src/threads/synch.h
@@ -41,6 +41,23 @@ void cond_wait (struct condition *, struct lock *);
 void cond_signal (struct condition *, struct lock *);
 void cond_broadcast (struct condition *, struct lock *);
 
+/* Readers-writers lock.
+
+   Implementation of "First readers-writers problem" from
+   https://en.wikipedia.org/wiki/Readers%E2%80%93writers_problem. */
+struct rwlock
+  {
+    struct semaphore resource;
+    struct lock rmutex;
+    unsigned readcount;
+  };
+
+void rwlock_init (struct rwlock *);
+void rwlock_write_p (struct rwlock *);
+void rwlock_write_v (struct rwlock *);
+void rwlock_read_p (struct rwlock *);
+void rwlock_read_v (struct rwlock *);
+
 /* Optimization barrier.
 
    The compiler will not reorder operations across an
-- 
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