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Concurrent Systems/notes/7- MUTEX-free concurrency.md

@@ -144,7 +144,15 @@ This is needed for the so called ABA problem with compare&set:
 	if X.compare&set(tmp, v) then ...
 ```
 - this is to ensure that the value of X has not changed in the computation
-- the problem is that X can be changed twice before compare&set
+- the problem is that X can be changed twice before compare&set (e.g. it was A, it became B and than came back to A)
 - solution: X is a pair ⟨val , seq_numb⟩, with the constraint that each modification of X increases its sequence_number
 	- with the compare&set you mainly test that the sequence_number has not changed
 
+TOP : a register that can be read or compare&setted
+	![[Pasted image 20250324100652.png]]
+
+```
+push(w) :=
+	while true do ⟨i,v,s⟩ <- TOP
+	conclude(i,v,s) if i=k then return FULL newtop ß ⟨i+1,w,STACK[i+1].seq_num+1⟩ if TOP.compare&set(⟨i,v,s⟩,newtop) then return OK
+```