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CSC369 – tutorial 4

CSC369 – tutorial 4. TA: Trevor Brown Slides: http://www.cs.utoronto.ca/~tabrown/csc369/week4.ppt. UNIX-like Signals. The data structure. In Unix, the signal number represents a bit position in a 32-bit flag in the thread struct Allows a thread to be sent multiple types of signals

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CSC369 – tutorial 4

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  1. CSC369 – tutorial 4 TA: Trevor Brown Slides: http://www.cs.utoronto.ca/~tabrown/csc369/week4.ppt

  2. UNIX-like Signals

  3. The data structure • In Unix, the signal number represents a bit position in a 32-bit flag in the thread struct • Allows a thread to be sent multiple types of signals • Positives? • minimal memory space, no dynamic memory allocation needed • just setting a bit is fast… • Negatives? • Signals can be lost. How? • … if two of the same type are sent before the first is handled. UNIX-like Signals

  4. An alternative design? • Create signal queue for each thread • linked list of structs containing a field that identifies the signal type • Get reliable signal delivery, but… • Downsides: • more memory • more time to allocate a struct and queue it up • What should you do? • You are free to design your own mechanism, but • For the signals we are supporting, losing a repeat is not a big deal. • Most people should prefer the simpler design. UNIX-like Signals

  5. The algorithm – how does it work? • How do we send signals? • Using the sys_kill() system call • The signaler (the thread that call sys_kill()): • Set a bit in the target thread • Error check and return • What kinds of things do we do to error check? • Returning: copyin/out • The signaled thread: • Checks its signal flag before returning to user space • Handles any signals that are marked UNIX-like Signals

  6. Some examples of signals • SIGKILL • Order a thread to roll over and die • SIGSTOP • Order a thread to go to sleep • Sleeping beauty style: must wait for prince SIGCONT to wake • SIGCONT • Wake up a thread put to sleep with SIGSTOP UNIX-like Signals

  7. Funny cases and design decisions • Funny cases are design decisions • They should appear in your design document! • What else should appear in your design doc.? • Any time you choose among several algorithms, why? • How does a thread wait for SIGCONT after getting SIGSTOP? • One way: use a condition variable • Recall, a condition variable supports operations: • wait() (suspend the invoking process and release the lock) • signal() (resume exactly one suspended process) • broadcast() (resumes all suspended processes) • We said a signaled thread checked and handles signals before returning to user space… • What if a thread receives a signal in kernel space? • Should a thread immediately die if it receives SIGKILL? UNIX-like Signals

  8. Assignment 2

  9. What are you doing in A2? • In a nut-shell: • Check out the new code (“…/a2” instead of “…/a1”) • Implementing parts of the PID system • noting that pid.c is a monitor which protects access to the PID array! • Implementing missing synchronization functions • thread_join • thread_detach • thread_exit • thread_fork • These must work together! • Implementing missing system calls • getpid • waitpid • kill Assignment 2

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  15. Some tips for A2 • Look at the new code in kern/thread • Search functionality in an IDE (e.g., NetBeans) makes browsing code (and finding functionality) much easier • Be very thorough. If one person implements, the other should code-read and test. • Read the man pages so you understand the parameters received and errors returned. • Reminders: • pid.c serves as a monitor to an important data structure • getpid and waitpid are user level tools • thread_join and thread_detach are kernel level tools • http://www.cdf.toronto.edu/~csc369h/fall/assignments/a2/a2.shtml Assignment 2

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