master-degree-notes/Concurrent Systems/notes/1b - Peterson algorithm.md

### Peterson algorithm (for two processes)
Let's try to enforce MUTEX with just 2 processes.

##### 1st attempt:
```
lock(i) :=
	AFTER_YOU <- i
	wait AFTER_YOU != i
	return

unlock(i) :=
	return
```
This protocol satisfies MUTEX, but suffers from deadlock (if one process never locks).

##### 2nd attempt:
```
Initialize FLAG[0] and FLAG[1] to down
lock(i) :=
	FLAG[i] <- up
	wait FLAG[1-i] = down
	return

unlock(i) :=
	FLAG[i] <- down
	return
```
Still suffers from deadlock if both processes simultaneously raise their flag.

##### Correct solution:
```
Initialize FLAG[0] and FLAG[1] to down
lock(i) :=
	FLAG[i] <- up
	AFTER_YOU <- i
	wait FLAG[1-i] = down OR AFTER_YOU != i
	return

unlock(i) :=
	FLAG[i] <- down
	return
```

**Features:**
- it satisfies MUTEX
- it satisfies bounded bypass, with bound = 1
- it requires 2 one-bit SRSW registers (the flags and 1 one-bit MRMW registers (AFTER_YOU)
- Each lock-unlock requires 5 accesses to the registers (4 for lock and 1 for unlock)

##### MUTEX proof
>[!info] 
>Remember that, in the [[#Correct solution|correct implementation]], there is this line: 	`wait (FLAG[1-i] = down OR AFTER_YOU != i)`. Thus, to access the critical section (the `return` instruction of the `lock` function), the possibilities are just the two discussed above.

**How has p0 entered its CS?**
a) `FLAG[1] = down`, this is possible only with the following interleaving:
![[Pasted image 20250303100721.png]]

b) `AFTER_YOU = 1`, this is possible only with the following interleaving:
![[Pasted image 20250303100953.png]]

##### Bounded Bypass proof (with bound = 1)
- If the wait condition is true, then it wins (and waits 0).

- Otherwise, it must be that `FLAG[1] = UP` **AND** `AFTER_YOU = 0` (quindi p1 ha invocato il lock e p0 dovrà aspettare).

- If p1 never locks anymore then p0 will eventually read `F[1]` and win (waiting 1).

- It is possible tho that p1 locks again:
	- if p0 reads `F[1]` before p1 locks, then p0 wins (waiting 1)
	- otherwise p1 sets A_Y to 1 and suspends in its wait (`F[0] = up AND A_Y = 1`), p0 will eventually read `F[1]` and win (waiting 1).

### Peterson algorithm ($n$ processes)
- FLAG now has $n$ levels (from 0 to n-1)
	- level 0 means down
	- level >0 means involved in the lock
- Every level has its own AFTER_YOU

```
Initialize FLAG[i] to 0, for all i

lock(i) :=
	for lev = 1 to n-1 do
		FLAG[i] <- lev
		AFTER_YOU[lev] <- i
		wait (∀k!=i, FLAG[k] < lev
			OR AFTER_YOU[lev] != i)
	return

unlock(i) :=
	FLAG[i] <- 0
	return
```
We say that: $p_i$ is at level $h$ when it exits from the $h$-th wait $\to$ a process at level $h$ is at any level $<= h$

>[!info] What is the `wait` condition actually checking?
>In the wait condition we check that all other processes are at a lower level.
##### MUTEX proof
>[!def] Lemma
>For every $ℓ \in \{0,\dots,n-1\}$ , at most n-ℓ processes are at level ℓ, this implies MUTEX by taking ℓ = n-1

Proof by induction on ℓ

Base (ℓ=0): trivial

Induction (true for ℓ, to be proved for ℓ+1):
- p at level ℓ can increase its level by writing its FLAG at ℓ+1 and its index in $A_Y[ℓ+1]$
- let $p_x$ be the last one that writes `A_Y[ℓ+1]`, so `A_Y[ℓ+1]=x`
- for $p_x$ to pass at level ℓ+1, it must be that $∀k≠x. F[k] < ℓ+1$, then $p_x$ is the only proc at level ℓ+1 and the thesis holds, since 1<=n-ℓ-1
- otherwise, $p_x$ is blocked in the wait and so we have at most n-ℓ-1 processes at level ℓ+1: those at level ℓ, that by induction are at most n-ℓ, except for px that is blocked in its wait.

##### Starvation freedom proof
**Lemma:** every process at level ℓ ($\leq n-1$) eventually wins $\to$ starvation freedom holds by taking $ℓ=0$.

Reverse induction on ℓ
Base ($ℓ=n-1$): trivial

Induction (true for ℓ+1, to be proved for ℓ):
- Assume a $p_x$ blocked at level ℓ (aka blocked in its ℓ+1-th wait) $\to \exists k\neq x, F[k]\geqℓ+1 \land A\_Y[ℓ+1]=x$

- If some $p_{y}$ will eventually set $A\_Y[ℓ+1]$ to $y$, then $p_x$ will eventually exit from its wait and pass to level ℓ+1

- Otherwise, let $G = \{p_{i}: F[i] \geq ℓ+1\}$ and $L=\{p_{i}:F[i]<ℓ+1\}$
	- if $p \in L$, it will never enter its ℓ+1-th loop (as it would write $A_Y[ℓ+1]$ and it will unblock $p_x$, but we are assuming that it is blocked)
	- all $p \in G$ will eventually win (by induction) and move to L
		- $\to$ eventually, $p_{x}$ will be the only one in its ℓ+1-th loop, with all the other processes at level <ℓ+1
		- $\to$ $p_{x}$ will eventually pass to level ℓ+1 and win (by induction)


##### Peterson algorithm cost
- $n$ MRSW registers of $\lceil \log_{2} n\rceil$ bits (FLAG)
- $n-1$ MRMW registers of $\lceil \log_{2}n \rceil$ bits (AFTER_YOU)
- $(n-1)\times(n+2)$ accesses for locking and 1 access for unlocking

It satisfies MUTEX and starvation freedom. It does not satisfy bounded bypass:
- consider 3 processes, one sleeping in its first wait, the others alternating in the CS
- when the first process wakes up, it can pass to level 2 and eventually win
- but the sleep can be arbitrary long and in the meanwhile the other two processes may have entered an unbounded number of CSs

Easy to generalize to k-MUTEX.