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

Critical sections (locks) have drawbacks:

• if not put at the right level of granularity, they unnecessarily reduce concurrency
• delays of one process may affect the whole system

MUTEX-freedom: the only atomicity is the one provided by the primitives themselves (no wrapping of code into CSs) the liveness properties used so far cannot be used anymore, since they rely on CSs. (example: if we have only atomic R/W registers, these are the only atomic things that we have. But we may also have atomic primitives like test&set, compare&swap ecc.).

Liveness properties

We have four new liveness properties

1. Obstruction freedom: every time an operation is run in isolation (no overlap with any other operation on the same object), it terminates
2. Non-blocking: whenever an operation is invoked on an object, eventually one operation on that object terminates
   • reminds deadlock-freedom in MUTEX-based concurrency
3. Wait freedom: whenever an operation is invoked on an object, it eventually terminates
   • reminds starvation-freedom in MUTEX-based concurrency
4. Bounded wait freedom: wait freedom + a bound on the number of steps needed to terminate
   • reminds bounded bypass in MUTEX-based concurrency

REMARK: these notions naturally cope with (crash) failures. Fail stop is another way of terminating, there is no way of distinguishing a failure from an arbitrary long sleep (because of asynchrony).

A wait-free Splitter

Assume we have atomic R/W registers.

A splitter is a concurrent object that provides a single operation dir such that:

1. (validity) it returns L, R or S (left, right, stop)
2. (concurrency) in case of n simultaneous invocations of dir
   1. at most n-1 L are returned
   2. at most n-1 R are returned
   3. at most 1 S is returned
3. (wait freedom) it eventually terminates.

Idea:

• not all processes obtain the same value
• in a solo execution (i.e., without concurrency) the invoking process must stop (0 L && 0 R && at most 1 S)

We have:

• DOOR: MRMW boolean atomic register initialized at 1
• LAST: MRMW atomic register initialized at whatever process index

dir(i) :=
	LAST <- i
	if DOOR = 0 then
		return R
	else
		DOOR <- 0
	if LAST = i then
		return S
	else
		return L

With 2 processes, we can have:

• one goes left and one goes right
• one goes left and the other stops
• one goes right and the other stops

Soundness theorem

this implementation satisfies the three requirements for the splitter

Proof:

1. Not all processes can obtain R
   • the door must have been closed and who closed the door cannot obtain R
2. not all processes can obtain L
   • let us consider the last process that writes into LAST (this is an atomic register, so this is meaningful)
   • if the door is closed, it receives R and √
3. let p_i be the first process that receives S \to LAST=i in its second if !

An Obstruction-free Timestamp Generator

A timestamp generator is a concurrent object that provides a single operation get_ts such that:

1. (validity) not two invocations of get_ts return the same value
2. (consistency) if one process terminates its invocation of get_ts before another one starts, the first receives a timestamp that is smaller than the one received by the second one
3. (obstruction freedom) if run in isolation, it eventually terminates

Idea: use something like a splitter for possible timestamp, so that only the process that receives S (if any) can get that timestamp.

We have:

• DOOR[i]: MRMW boolean atomic register initialized at 1, for all i
• LAST[i]: MRMW atomic register initialized at whatever process index, for all i
• NEXT: integer initialized at 1

get_ts(i) :=
	k <- NEXT
	while true do
		LAST[k] <- i
		if DOOR[k] = 1 then
			DOOR[k] <- 0
			if LAST[k] = i then
				NEXT++
				return k
			k++

Soundness theorem (yes, again)

this implementation satisfies the three properties of the timestamp generator

1. Validity holds because of property 2.c of the splitter
2. For consistency, the invocation that terminates increased the value of NEXT before terminating
   • every process that starts after its termination will find NEXT to a greater value (NEXT never decreases!)
3. Obstruction freedom is trivial

REMARK: this implementation doesn’t satisfy the non-blocking property:!

A Wait-free Stack

REG is an unbounded array of atomic registers (the stack)

For all i, REG[i] can be:

• written
• read by the swap(v) primitives (that atomically writes a new value in it)
• initialized at ⊥

NEXT is an atomic register (pointing at the next free location of the stack) that can be

• read
• fetch&add
• initialized at 1

push(v) :=
	i <- NEXT.fetch&add(1)
	REG[1] <- v

pop() :=
	k <- NEXT-1
	for i = k down to 1
	tmp <- REG[1].swap(⊥)
	if tmp != ⊥ then
		return tmp
	return ⊥

REMARK: crashes do not compromise progress! PROBLEM: unboundedness of REG is not realistic

A Non-blocking Bounded Stack

Idea: every operation is started by the invoking process and finalized by the next process

STACK[0...k] : an array of registers that can be read or compare&setted STACK[i] is actually a pair ⟨val , seq_numb⟩ initialized at ⟨⊥,0⟩

This is needed for the so called ABA problem with compare&set:

• A typical use of compare&set is

	tmp <- X
	...
	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 (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 !

push(w) :=
	while true do
		⟨i,v,s⟩ <- TOP
		conclude(i,v,s)
		if i=k then
			return FULL
			newtop <- ⟨i+1,w,STACK[i+1].seq_num+1⟩
			if TOP.compare&set(⟨i,v,s⟩,newtop) then
				return OK

conclude(i, v, s) :=
	tmp <- STACK[i].val
	STACK[i].compare&set(⟨tmp,s-1⟩,⟨v,s⟩)

pop() :=
	while true do
		⟨i,v,s⟩ <- TOP
		conclude(i,v,s)
		if i=0 then
			return EMPTY
	newtop <- ⟨i-1,STACK[i-1⟩
	if TOP.compare&set(⟨i,v,s⟩,newtop) then
		return v

Liveness theorem

the implementation of the stack is non-blocking

Proof: mi fido