I have read a lot of documentation and ran experiments in the TLA+ toolbox to test how labels work.
I know that it meant to identify units of atomic execution, but I discovered that it doesn't work entirely as I expect.
Here are a few cases I would like to understand:
Assume I have two fair processes A and B, and that I have a globally defined seq cache = <<>>.
Each process has label_1 that performs an operation on the cache.
Q1: what does atomicity mean in this case? can both processes access the cache at the same time?
Q2: I want to be able to loop in an atomic fashion, why does the label apply to a single iteration of the while loop?
Q3: If a process contains for example 3 labels, lb1, lb2, lb3, will that process attempt to execute them sequentially?
I implemented mutex to lock resources with the intent that I can lock a resource in one label and release in the next label (to mimic a prepare phase on an entity), but I discovered that sometimes the process skips the first label and goes directly to the second label. For example:
while some condition:
if some condition:
\* do something
I was sure that it will never skip to L_Gen_Version since it is defined within the if statement. However, I saw that the process translated to TLA+ as follow: A = While_label \/ L_Gen_Version, and sometimes when the condition of the if is not satisfied it jumps to L_Gen_Version.
I am very confused by this behavior.
Clarification of the labels behavior will be greatly appreciated.
If it is important I implemented mutex as following:
macro cache_lock() begin
await c_lock = 0;
c_lock := 1;
macro cache_unlock() begin
c_lock := 0;