System and device programming——R&W with semaphore

Posted 2020-06-23

Readers Precedence

when both readers and writers wait,favor readers and writers can write only after all readers read.Using semaphore me1 to let writer need to wait one more signal.e.g.when 

writer w1 enters and no readers,w1 can write,then have both writers and readers are waiting. Reader r1 can continue till wait(w),but writer w2 can‘t enter because it needs w1

completes to release semaphore(me1).After w1 finishes writing,it releases semaphore w first,then reader r1 gets immediately. Then reader r1 increases counter,and release semaphore me later which can let next reader increases counter,in this way till last reader

finishes reading and decreases(nr==0) counter,semaphore w can only be released.Only after this,can writer gets semaphore w and writes.

nr=0(number of readers currently reading)

int(w)=int(me)=int(me1)=1

Reader                  Writer

wait(me);                wait(me1)

 nr++;                   wait(w)

 if(nr==1) wait(w)          ...

signal(me)                //writing

...                     ...

//reading                  signal(w)

...                    signal(me1)

wait(me)

 nr--;

 if(nr==0) signal(w)

signal(me)

Writers Precedence

The use of me2 is same with previous case

nr=nw=0;init(r)=init(w)=init(me)=init(me1)=init(me2)=1

Reader                               Writer

wait(me2)                             wait(me1)

 wait(r)                              nw++

//needs to see whether there‘s writer            if(nw==1) wait(r)

//waiting,previous only first reader needs to check signal(me1)

//and checks only writing                  wait(w)

  wait(me)                            .

  nr++                               .

   if(nr==1) wait(w)                    //Writing

  signal(me)                           .

 signal(r)                            .

signal(me2)                            signal(w)

 //Reading                            wait(me1)

wait(me)                                nw--

 nr--                                 if(nw==0)signal(r)

 if(nr==0) signal(w)                     signal(me1)

signal(me2)

 

when no precedence for both readers and writers,but first in,first served,what to do?

maybe list