Reader-Writer-Problem

Ref: https://arxiv.org/pdf/1309.4507.pdf

Sempahore(n)
Decrement opeartion => wait
Increment operatorn => signal

mutex = sempaphore(1)

Classical Solution
init:
mx = sempaphore(1)
wrt = sempahore(1)
ctr = integer(0)

reader:
begin
wait mx
if (++ctr) == 1, then wait wrt
singal mx
[critical section of reader]
wait mux
if (--ctr)==0, then signal wrt
signal mx
end

writer:
begin
wait wrt
[critical section
signal wrt

Note: downsite: startvation of the writer.
A writer read does not have a chance to execute while any num readers continuously entering and and leaving the working area.


2nd Solution
init:
in = Semaphore(1)
mx = sempaphore(1)
wrt = sempahore(1)
ctr = integer(0)

reader:
begin
wait in
wait mx
if (++ctr) == 1, then wait wrt
singal mx
signal in
[critical section of reader]
wait mux
if (--ctr)==0, then signal wrt
signal mx
end

writer:
begin
wait in
wait wrt
[critical section
signal wrt
singal in

------------
3rd Solution
-------------
init:
in = Semaphore(1)
out = semaphore(1)
wrt = sempahore(1)
ctrin = integer(0)
ctrout = integer(0)
wait = boolean(0)

reader:
begin
wait in
ctrin++
singal in
[critical section of reader]
wait mux
ctrout++
// last reader signals writer that is is safe to proceed
if (wait==1 && ctrin==ctrout) then signal wrt
signal out
end

writer:
begin
wait in
wait out
if (ctrin == ctrout) {
  signal out
} else {
  wait = 1
  signal out
  wait wrt
  wait=0
}
critical section
singal in