vault backup: 2025-03-12 11:39:27

Concurrent Systems/notes/4 - Semaphores.md

@@ -180,3 +180,42 @@ begin_write() :=
 end_write() :=
 	GLOB_MUTEX.up()
 	return
+```
+
+##### Strong priority to Readers
+When a writer terminates, it activates the first reader, if there is any, or the first writer, otherwise.
+
+```
+GLOB_MUTEX and R_MUTEX and W_MUTEX semaphores init. at 1
+R a shared register init. at 0
+
+begin_read() :=      end_read() :=
+	come prima           come prima
+
+begin_write() :=
+	W_MUTEX.down()
+	GLOB_MUTEX.down()
+	return
+
+end_write() :=
+	GLOB_MUTEX.up()
+	W_MUTEX.up()
+	return
+```
+
+##### Weak priority to Writers
+```
+GLOB_MUTEX, PRIO_MUTEX, R_MUTEX and W_MUTEX semaphores init. at 1 R and W shared registers init. at 0
+
+begin_read() :=
+	PRIO_MUTEX.down()
+	R_MUTEX.down()
+	R++
+	if R = 1 then
+		GLOB_MUTEX.down()
+	R_MUTEX.up()
+	PRIO_MUTEX.up()
+	return
+
+end_read() :=
+