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Concurrent Systems/notes/4 - Semaphores.md

@@ -127,7 +127,32 @@ B.consume() :=
 Thanks to the semaphores, we are sure that while loops will not go on forever! The loops starts only if there is at least a FREE / BUSY cell.
 
 #### (Multiple) Producers/Consumers - Wrong solution
-EXERCISE - will do later
+**EXERCISE** this example is wrong
+```
+B.produce(v) :=
+	FREE.down()
+	SP.down()
+	i <- IN
+	IN <- (IN + 1) mod k
+	EMPTY[in] <- ff
+	SP.up()
+	BUF[i] <- v
+	FULL[i] <- tt
+	BUSY.up()
+	return
+
+B.consume() :=
+	BUSY.down()
+	SC.down()
+	o <- OUT
+	OUT <- (OUT+1) mod k
+	FULL[OUT] <- ff
+	SC.up()
+	tmp <- BUF[o]
+	EMPTY[o] <- tt
+	FREE.up()
+	return tmp
+```
 
 #### The Readers/Writers problem
 - Several processes want to access a file
@@ -181,7 +206,7 @@ end_write() :=
 	GLOB_MUTEX.up()
 	return
 ```
-If readers keeps arriving, they surpass writers. But when a writer terminates a 
+If readers keeps arriving, they surpass writers. But when a writer terminates it activates the first suspended process, which can be a reader or a writer.
 ##### Strong priority to Readers
 When a writer terminates, it activates the first reader, if there is any, or the first writer, otherwise.
 
@@ -244,3 +269,4 @@ def end_write() :=
 	return
 ```
 This is prioritizing writers as if there are writers waiting, they will be waiting at `GLOB_MUTEX.down()`. This semaphore is upped before `PRIO_MUTEX` which is the one that blocks readers.
+But writers won't be able to writer until there are no readers, if they keep coming, they will block the writers as `GLOB_MUTEX` will never be upped.