1. CSC 660: Advanced Operating SystemsSlide #1 CSC 660: Advanced OS Synchronization

2. CSC 660: Advanced Operating SystemsSlide #2 Topics 1.Race Conditions 2.Critical Sections 3.Kernel Concurrency 4.What needs Protection? 5.Atomic Operations 6.Spin Locks 7.Semaphores 8.Deadlock

3. CSC 660: Advanced Operating SystemsSlide #3 Race Conditions abc.pdf prog.c as2.txt 4 5 7 6 out = 4 in = 7 Process A Process B

4. CSC 660: Advanced Operating SystemsSlide #4 Race Conditions 1.Process A read in, stores in local var slot. 2.Timer interrupt. 3.Scheduler switches to process B. 4.Process B reads in, stores in local var slot. 5.Process B stores filename in slot 7 (slot). 6.Process B updates in to be 8. 7.Scheduler eventually switches to process A. 8.Process A writes filename in slot 7 (slot). 9.Process A computes in = slot + 1. 10.Process A updates in to be 8.

5. CSC 660: Advanced Operating SystemsSlide #5 Critical Sections How can we prevent race conditions? –Prohibit multiple processes from accessing shared resource at the same time. –Critical section is code that accesses shared resource.

6. CSC 660: Advanced Operating SystemsSlide #6 Critical Sections 1.No two processes may be simultaneously inside their critical sections. 2.No assumptions may be made about speed or number of CPUs. 3.No process running outside its critical section may block other processes. 4.No process should have to wait forever to enter its critical section.

7. CSC 660: Advanced Operating SystemsSlide #7 Critical Sections

8. CSC 660: Advanced Operating SystemsSlide #8 Kernel Concurrency 1.Interrupts 2.Softirqs and tasklets 3.Kernel preemption 4.Sleeping 5.SMP

9. CSC 660: Advanced Operating SystemsSlide #9 What needs Protection? What needs protection? –Global kernel data structures. –Shared data between process/interrupt context. –Shared data between interrupt handlers. What doesn’t? –Data local to executing thread (i.e., stack.) –Local variables. –Dynamically allocated data w/ only local ptrs. –Per-CPU data doesn’t need SMP protection.

10. CSC 660: Advanced Operating SystemsSlide #10 Why do we need atomicity? Process A read i(7) incr i(7 -> 8) - write i(8) Process B read i(7) - incr i(7 -> 8) - write i(8) Problem: Two processes incrementing i. A: read i(7) A: incr i(7 -> 8) B: read i(8) B: incr i(8 -> 9) Uniprocessor Version

11. CSC 660: Advanced Operating SystemsSlide #11 Atomic Operations Process A atomic_inc i (7->8) Process B - atomic_inc i (8->9) Atomic operations are indivisible. Provided by atomic_t in the kernel. Operations: atomic_{read,set,add,sub,inc,dec,dec_and_test) x86 assembly: lock byte preceding opcode makes atomic.

12. CSC 660: Advanced Operating SystemsSlide #12 Atomicity doesn’t provide Ordering Process A atomic_inc i (7->8) Process B - atomic_inc i (8->9) One atomic order of operations: Another atomic order of operations: Process A - atomic_inc i (8->9) Process B atomic_inc i (7->8)

13. CSC 660: Advanced Operating SystemsSlide #13 Barriers provide Ordering Optimization Barriers –Prevent compiler from re-ordering instructions. –Compiler doesn’t know when interrupts or other processors may read/write your data. –Kernel provides barrier() macro. Memory Barriers –Read/write barriers prevent loads/stores from being re-ordered across barrier. –Kernel provides rmb(), wmb() macros.

14. CSC 660: Advanced Operating SystemsSlide #14 Spin Locks If lock “open” –Sets lock bit with atomic test-and-set. –Continues into critical region. else lock “closed” –Code “spins” in busy wait loop until available. –Kernel-preemption or SMP will interrupt.

15. CSC 660: Advanced Operating SystemsSlide #15 Spin Lock Functions spin_lock_init(spinlock_t *lock) Initialize spin lock to 1 (unlocked). spin_lock(spinlock_t *lock) Spin until lock becomes 1, then set to 0 (locked). spin_lock_irqsave(spinlock_t *l, u flags) Like spin_lock() but disables and saves interrupts. Always use an IRQ disabling variant in interrupt context. spin_unlock(spinlock_t *lock) Set spin lock to 1 (unlocked). spin_lock_irqrestore(spinlock_t *l, u flags) Like spin_lock(), but restores interrupt status. spin_trylock(spinlock_t *lock) Set lock to 0 if unlocked and return 1; return 0 if locked.

16. CSC 660: Advanced Operating SystemsSlide #16 Spin Lock 1.Disables kernel pre-emption. 2.Atomic test-and-sets lock. 3.If old value positive Lock acquired. 4.Else Enables pre-emption. If break_lock is 0, sets to 1 to indicate a task is waiting. Busy wait loop while (spin_is_locked(lock)) cpu_relax(); # pause instruction on P4 Goto 1.

17. CSC 660: Advanced Operating SystemsSlide #17 Semaphores Integer value with atomic access. If S>0, semaphore prevents access. Using a semaphore for mutual exclusion: down(S); /* critical section */ up(S);

18. CSC 660: Advanced Operating SystemsSlide #18 Semaphores Down (P): Request to enter critical region. If S > 0, decrements S, enters region. Else process sleeps until semaphore is released. Up (V): Request to exit critical region. Increments S. If S > 0, wakes sleeping processes.

19. CSC 660: Advanced Operating SystemsSlide #19 Linux Semaphores DECLARE_MUTEX(sem); Static declares a mutex semaphore. void init_MUTEX(struct semaphore *sem); Dynamic declaration of a mutex semaphore. void down(struct semaphore *sem); Decrements semaphore and sleeps. int down_interruptible(struct semaphore *sem); Same as down() but returns on user interrupt. int down_trylock(struct semaphore *sem); Same as down() but returns immediately if not available. void up(struct semaphore *sem); Releases semaphore.

20. CSC 660: Advanced Operating SystemsSlide #20 Linux Semaphores #include struct semaphore sem; init_MUTEX(&sem); if (down_interruptible(&sem)) return –ERESTARTSYS; /* user interrupt */ /* * critical section */ up(&sem);

21. CSC 660: Advanced Operating SystemsSlide #21 Tradeoffs Spin Locks Busy waits waste CPU cycles. Semaphores Context switch on sleep is expensive.

22. CSC 660: Advanced Operating SystemsSlide #22 Deadlocks Process A down(A) down(B) Process B down(B) down(A) Single process deadlock: A: spin_lock(A) Two process deadlock:

23. CSC 660: Advanced Operating SystemsSlide #23 Deadlock Prevention Lock Ordering If you acquire multiple locks, you must always acquire them in the same order. Don’t double acquire a lock. Always use interrupt-disabling variants of spin_lock() in interrupt context. Always release locks. Be sure that your code will always make appropriate calls to release locks.

24. CSC 660: Advanced Operating SystemsSlide #24 Key Points Synchronization is essential for accessing shared kernel data structures. Kernel provides two major types: –Spin Locks –Semaphores –and many variants. System performance depends strongly on the type of synchronization used. Synchronization must be used carefully and consistently to avoid deadlocks.

25. CSC 660: Advanced Operating SystemsSlide #25 References 1.Daniel P. Bovet and Marco Cesati, Understanding the Linux Kernel, 3 rd edition, O’Reilly, 2005. 2.Johnathan Corbet et. al., Linux Device Drivers, 3 rd edition, O’Reilly, 2005. 3.Robert Love, Linux Kernel Development, 2 nd edition, Prentice-Hall, 2005. 4.Claudia Rodriguez et al, The Linux Kernel Primer, Prentice-Hall, 2005. 5.Peter Salzman et. al., Linux Kernel Module Programming Guide, version 2.6.1, 2005. 6.Andrew S. Tanenbaum, Modern Operating Systems, 3 rd edition, Prentice-Hall, 2005.