Tutorial Week 3


    System Call Interface

  1. The following segment of code is similar (but much simpler) to the main task that the daemon inetd performs. It accepts connections on a socket and forks a process to handle the connection.

    This is not guaranteed to be compilable. Use the man command if you want to investigate what all the system calls are doing.

    0001 xxx(int socket){
    0003        while ((fd = accept(socket, NULL, NULL)) >= 0) {
    0004                switch((pid = fork())) {
    0005                case -1:
    0006                        syslog(LOG_WARN, "%s cannot create process: %s",
    0007				progname, sys_error(errno));
    0008                        continue;
    0009                case 0:         
    0010                        close(0);
    0011                        close(1);
    0012                        dup(fd);
    0013                        dup(fd);
    0014                        execl("/usr/sbin/handle_connection", 
    0015				"handle_connection", NULL);
    0016                        syslog(LOG_WARN, "%s cannot exec handle_connection\ 
    0017				helper : %s", progname, sys_error(errno));
    0018                        _exit(0);
    0019                default: 
    0020                        waitpid(pid, &status, 0); 
    0021                        if (WIFEXITED(status) && WIFEXITSTATUS(status) == 0)
    0022                                continue;
    0023                        syslog(LOG_WARN, "handle_connection failed:\
    0024				exit status +%d\n", status);
    0025                }
    0026        }
    0027 }
    1. Identify which lines of code are executed by the parent process.
    2. Identify which lines of code are invoked by the child process.
    3. Under what circumstances does the child terminate?

  2. Concurrency and Deadlock

  3. For each of the following scenarios, one or more dining philosophers are going hungry. What is the condition the philosophers are suffering from?

    1. Each philosopher at the table has picked up his left fork, and is waiting for his right fork
    2. Only one philosopher is allowed to eat at a time. When more than one philosophy is hungry, the youngest one goes first. The oldest philosopher never gets to eat.
    3. Each philosopher, after picking up his left fork, puts it back down if he can't immediately pick up the right fork to give others a chance to eat. No philosopher is managing to eat despite lots of left fork activity.

  4. What is starvation, give an example?

  5. Two processes are attempting to read independent blocks from a disk, which involves issuing a seek command and a read command. Each process is interrupted by the other in between its seek and read. When a process discovers the other process has moved the disk head, it re-issues the original seek to re-position the head for itself, which is again interrupted prior to the read. This alternate seeking continues indefinitely, with neither process able to read their data from disk. Is this deadlock, starvation, or livelock? How would you change the system to prevent the problem?

  6. Describe four ways to prevent deadlock by attacking the conditions required for deadlock.

  7. Answer the following questions about the tables.

    1. Compute what each process still might request and display in the columns labeled "still needs".
    2. Is the system in a safe or unsafe state? Why?
    3. Is the system deadlocked? Why or why not?
    4. Which processes, if any, are or may become deadlocked?
    5. Assume a request from p3 arrives for (0,1,0,0)
      1. Can the request be safely granted immediately?
      2. In what state (deadlocked, safe, unsafe) would immediately granting the request leave the system?
      3. Which processes, if any, are or may become deadlocked if the request is granted immediately?
    r1 r2 r3 r4
    2 1 0 0
    current allocation maximum demand       still needs    
    process r1 r2 r3 r4 r1 r2 r3 r4 r1 r2 r3 r4
    p1 0 0 1 2 0 0 1 2
    p2 2 0 0 0 2 7 5 0
    p3 0 0 3 4 6 6 5 6
    p4 2 3 5 4 4 3 5 6
    p5 0 3 3 2 0 6 5 2

  8. R3000 and assembly

  9. What is a branch delay?

  10. The goal of this question is to have you reverse engineer some of the C compiler function calling convention (instead of reading it from a manual). The following code contains 6 functions that take 1 to 6 integer arguments. Each function sums its arguments and returns the sum as a the result.

    #include <stdio.h> /* function protoypes, would normally be in header files */ int arg1(int a); int arg2(int a, int b); int arg3(int a, int b, int c); int arg4(int a, int b, int c, int d); int arg5(int a, int b, int c, int d, int e ); int arg6(int a, int b, int c, int d, int e, int f); /* implementations */ int arg1(int a) { return a; } int arg2(int a, int b) { return a + b; } int arg3(int a, int b, int c) { return a + b + c; } int arg4(int a, int b, int c, int d) { return a + b + c + d; } int arg5(int a, int b, int c, int d, int e ) { return a + b + c + d + e; } int arg6(int a, int b, int c, int d, int e, int f) { return a + b + c + d + e + f; } /* do nothing main, so we can compile it */ int main() { }

    The following code is the disassembled code that is generated by the C compiler (with certain optimisations turned of for the sake of clarity).

    004000f0 <arg1>: 4000f0: 03e00008 jr ra 4000f4: 00801021 move v0,a0 004000f8 <arg2>: 4000f8: 03e00008 jr ra 4000fc: 00851021 addu v0,a0,a1 00400100 <arg3>: 400100: 00851021 addu v0,a0,a1 400104: 03e00008 jr ra 400108: 00461021 addu v0,v0,a2 0040010c <arg4>: 40010c: 00852021 addu a0,a0,a1 400110: 00861021 addu v0,a0,a2 400114: 03e00008 jr ra 400118: 00471021 addu v0,v0,a3 0040011c <arg5>: 40011c: 00852021 addu a0,a0,a1 400120: 00863021 addu a2,a0,a2 400124: 00c73821 addu a3,a2,a3 400128: 8fa20010 lw v0,16(sp) 40012c: 03e00008 jr ra 400130: 00e21021 addu v0,a3,v0 00400134 <arg6>: 400134: 00852021 addu a0,a0,a1 400138: 00863021 addu a2,a0,a2 40013c: 00c73821 addu a3,a2,a3 400140: 8fa20010 lw v0,16(sp) 400144: 00000000 nop 400148: 00e22021 addu a0,a3,v0 40014c: 8fa20014 lw v0,20(sp) 400150: 03e00008 jr ra 400154: 00821021 addu v0,a0,v0 00400158 <main>: 400158: 03e00008 jr ra 40015c: 00001021 move v0,zero
    1. arg1 (and functions in general) returns its return value in what register?
    2. Why is there no stack references in arg2?
    3. What does jr ra do?
    4. Which register contains the first argument to the function?
    5. Why is the move instruction in arg1 after the jr instruction.
    6. Why does arg5 and arg6 reference the stack?

  11. The following code provides an example to illustrate stack management by the C compiler. Firstly, examine the C code in the provided example to understand how the recursive function works.

    #include <stdio.h> #include <unistd.h> char teststr[] = "\nThe quick brown fox jumps of the lazy dog.\n"; void reverse_print(char *s) { if (*s != '\0') { reverse_print(s+1); write(STDOUT_FILENO,s,1); } } int main() { reverse_print(teststr); }

    The following code is the disassembled code that is generated by the C compiler (with certain optimisations turned off for the sake of clarity).

    1. Describe what each line in the code is doing.
    2. What is the maximum depth the stack can grow to when this function is called?
    004000f0 <reverse_print>: 4000f0: 27bdffe8 addiu sp,sp,-24 4000f4: afbf0014 sw ra,20(sp) 4000f8: afb00010 sw s0,16(sp) 4000fc: 80820000 lb v0,0(a0) 400100: 00000000 nop 400104: 10400007 beqz v0,400124 <reverse_print+0x34> 400108: 00808021 move s0,a0 40010c: 0c10003c jal 4000f0 <reverse_print> 400110: 24840001 addiu a0,a0,1 400114: 24040001 li a0,1 400118: 02002821 move a1,s0 40011c: 0c1000af jal 4002bc <write> 400120: 24060001 li a2,1 400124: 8fbf0014 lw ra,20(sp) 400128: 8fb00010 lw s0,16(sp) 40012c: 03e00008 jr ra 400130: 27bd0018 addiu sp,sp,24

  12. Why is recursion or large arrays of local variables avoided by kernel programmers?

  13. Threads

  14. Compare cooperative versus preemptive multithreading?

  15. Describe user-level threads and kernel-level threads. What are the advantages or disadvantages of each approach?

  16. A web server is constructed such that it is multithreaded. If the only way to read from a file is a normal blocking read system call, do you think user-level threads or kernel-level threads are being used for the web server? Why?

  17. Assume a multi-process operating system with single-threaded applications. The OS manages the concurrent application requests by having a thread of control within the kernel for each process. Such a OS would have an in-kernel stack assocaited with each process.

    Switching between each process (in-kernel thread) is performed by the function switch_thread(cur_tcb,dst_tcb). What does this function do?