vault backup: 2022-10-30 14:40:53

content/notes/20-sync-deadlock.md

@@ -12,8 +12,8 @@ tags:
 producer consumer problem
 
 issues
-- no data in buffe: consumer has nothing to consume
-- buffer is full: no space for producer to produce
+- no data in buffe: consumer has nothing to consume: data race
+- buffer is full: no space for producer to produce: busy waiting
 
 private vars
 - in: next pos in buf to be writted
@@ -24,4 +24,32 @@ shared variables
 - counter: number of items in buffer
 
 waiting
-- busy"
+- busy: proces keeps checking (if buffer is still full) wasting CPU time
+- non busy: suspend the process 
+
+soution with shared array
+```c
+PRODUCER: repeat 
+		...
+		produce an item in nextp 
+		...
+		while counter = n do no-op;
+		buffer[in] := nextp;
+		in := in+1 mod n;
+		counter := counter+1;
+	until false;	
+CONSUMER: repeat 
+		while counter = 0 do no-op;
+		nextc := buffer[out];
+		out := out+1 mod n;
+		counter := counter-1;
+
+		...
+		consume the item in nextc 
+		...
+	until false;
+```
+
+counter is a shared variable - causes a data race problem
+
+![data race problem slide|400](https://i.imgur.com/qIz6FGU.png)