RW1

Reader-Writer with Writer Starvation (From Peterson/Galvin)

Reader Entry                 Writer Entry

P(mutex);                         P(wsem)
  rc++;
  if rc=1 then P(wsem);
V(mutex);



Reader Exit                   Writer Exit

P(mutex);                         V(wsem)
  rc--;  
  if rc=0 then V(wsem);
V(mutex);

RW2

Reader-Writer with Reader Starvation

Reader Entry

P(rsem);
V(rsem)
P(rmutex);
 rc++
 if rc=1 then P(wsem);
V(rmutex);



Reader Exit

P(rmutex);
 rc--;
 if rc=0 then V(wsem);
V(rmutex);

Writer Entry

P(wmutex);
 wc++;
 if wc=1 then P(rsem);
V(wmutex);
P(wsem);



Writer Exit

V(wsem);
P(wmutex);
  wc--;
  if wc=0 then V(rsem);
V(wmutex);

RW3

Reader Writer Solution - I

Reader Entry
P(mutex);
   if (wwc>0) or (wc>0) then begin 
            rwc++;
            V(mutex);
            P(rsem);
            P(mutex);
            rwc--;  end;
  rc++;
V(mutex);


Reader Exit
P(mutex);
  rc--;
  if (rc=0) and (wwc>0) then V(wsem);
V(mutex);

Writer Entry
P(mutex);
if (rc>0)or(wc>0)or(rwc>0)or(wwc >0)
         then begin
            wwc++;
            V(mutex);
            P(wsem);
            P(mutex);
            wwc--; end;
 wc++;
 V(mutex);

Writer Exit
  P(mutex);
   wc-- ;   
   if (rwc>0) then 
      for i:=1 to rwc do V(rsem)
   else if (wwc>0) then V(wsem);
  V(mutex)

RW4

Reader Writer Solution - II

Reader Entry

P(mutex);
   if (wwc>0) or (wc>0) then begin 
            rwc++;
            V(mutex);
            P(rsem);
            rwc--; 
      end;
  rc++;
  if rwc>0 then V(rsem)
  else V(mutex);

Reader Exit

P(mutex);
  rc--;
  if (rc=0) and (wwc>0) then V(wsem);
  else V(mutex);

Writer Entry
P(mutex);
if (rc>0)or(wc>0)
         then begin
            wwc++;
            V(mutex);
            P(wsem);
            wwc--; 
         end;
    wc++;
 V(mutex);

Writer Exit
  P(mutex);
   wc-- ;   
   if (rwc>0) then V(rsem)
   else 
      if (wwc>0) then V(wsem);
      else V(mutex)