When will deadlock occur in this producer-consumer code

Question

I was reading about Producer Consumer problem from Operating Systems by William Stallings where following code was given:

+----------------------------------------------------+-------------------------------------------------------+
|               Producer                             |             Consumer                                  |
+------------------------------------------------------------------------------------------------------------+
| 1  int n;                                          | 1  void consumer()                                    |
| 2  binary_semaphore mx = 1, delay = 0;             | 2  {  semWaitB(delay);  //wait till first data item   |
| 3  void producer()                                 | 3                       //is produced                 |
| 4  {                                               | 4     while (true)                                    |
| 5     while (true)                                 | 5     {                                               |
| 6     {                                            | 6        semWaitB(mx); //continue if producer is not  |
| 7        produce();                                | 7                      //producing                    |
| 8        semWaitB(mx);  //continue if consumer     | 8        take();                                      |
| 9                       //is not consuming         | 9        n--;                                         |
| 10        append();                                | 10        semSignalB(mx);//signal done with consuming |
| 11        n++;                                     | 11        consume();                                  |
| 12        if (n==1) semSignalB(delay);  //unblocks | 12        if (n==0) semWaitB(delay);  //block self if |
| 13                                      //consumer | 13                                    //no data item  |
| 14        semSignalB(mx);  //signal done with      | 14     }                                              |
| 15                         //producing             | 15  }                                                 |
| 16     }                                           | 16  void main()                                       |
| 17  }                                              | 17  {  n = 0;                                         |
|                                                    | 18     parbegin (producer, consumer);                 |
|                                                    | 19  }                                                 |
+----------------------------------------------------+-------------------------------------------------------+

It then says that (referring to the line numbers in table below):

If consumer exhaust buffer setting n to 0 (line 8), producer has incremented it to 1 (line 11 of table), before consumer checking n and waiting on line 14. Line 14 should have blocked consumer since buffer was exhausted, but it did not, since producer incremented n meanwhile. Worst, consumer can immediately run again to consume non existent item to reduce n to -1 (line 20)

Then it says:

We cannot move the conditional statement inside the critical section as this could lead to deadlock (e.g. after line 8 of above table).

It continues to give different solution.

But I am not able to understand how it will lead to the deadlock. Considering following modified consumer code:

1  void consumer()
2  {  semWaitB(delay);  //wait till first data item
3                       //is produced
4     while (true) 
5     {
6        semWaitB(mx); //continue if producer is not 
7                      //producing
8        take();
9        n--;
10        if (n==0) semWaitB(delay);  //block self if
11                                    //no data item
12        semSignalB(mx);//signal done with consuming
13        consume();
14     }
15  }
16  void main()
17  {  n = 0;
18     parbegin (producer, consumer);
19  }

I came up with following:

As you can see, at the end, the values of mx, n and delay are reset to the ones before start. So how this could lead to deadlock? (In fact I feel this could be precise solution.)

Answer 1

It will definitely lead to deadlock. Consider the sequence of operations:

Producer successfully produces 1 item. This results in the following values of the sempaphores:

mx = 1 and delay = 1

Now consumer executes its code and reaches line 10

if (n==0) semWaitB(delay);

because delay is set to 0 because of line 2 of consumer

semWaitB(delay);

The line 10 will block the consumer and at this point we have mx = 0 because of line 6 of consumer semWaitB(mx);

Consumer is already blocked and producer will be blocked due to line 8 semWaitB(mx); as mx = 0. This is a deadlock