AOS Lab 4: If you liked it, then you should have put a “lock” on it

1. Lab 4: If you liked it, then you should have put a “lock” on it Advanced Operating Systems Zubair Nabi zubair.nabi@itu.edu.pk February 20, 2013

2. Concurrency within the OS 1 Multiple CPUs share kernel data structures • Can interfere with each other leading to inconsistencies 2 Even on uniprocessors, interrupt handlers can interfere with non-interrupt code 3 xv6 uses locks for both situations

5. Race conditions: Example • Several processors share a single disk • Disk driver has a single linked list idequeue of outstanding disk requests • Processors concurrently make a call to iderw which adds a request to the list

8. Race conditions (2): Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 struct list { int data; struct list ∗ next; }; struct list ∗ list = 0; void insert (int data) { struct list ∗ l; l = malloc ( sizeof ∗ l); l −>data = data; l −>next = list; list = l; }

9. Race conditions (3): Problem • Possible race condition on line 13 and 14 • In case of two concurrent insertions, the ﬁrst one will be lost

10. Race conditions (3): Problem • Possible race condition on line 13 and 14 • In case of two concurrent insertions, the ﬁrst one will be lost

11. Race conditions (4): Solution 1 2 3 4 5 6 7 8 9 10 11 12 13 struct list ∗ list = 0; struct lock listlock ; void insert (int data) { struct list ∗ l; acquire (& listlock ); l = malloc ( sizeof ∗ l); l −>data = data; l −>next = list; list = l; release (& listlock ); }

12. Lock representation 1 2 3 struct spinlock { uint locked ; };

13. Acquire lock 1 2 3 4 5 6 7 8 9 void acquire ( struct spinlock ∗ lk) { for (;;) { if(!lk −>locked ) { lk −>locked = 1; break ; } } }

14. Acquiring lock (2): Problem • Possible race condition on line 4 and 5 • So the solution itself causes a race condition!

15. Acquiring lock (2): Problem • Possible race condition on line 4 and 5 • So the solution itself causes a race condition!

16. Hardware support 1 2 3 4 5 6 7 8 9 10 static inline uint xchg( volatile uint ∗ addr , uint newval ) { uint result ; asm volatile ("lock; xchgl %0, %1" : "+m" ( ∗ addr), "=a" ( result ) : "1" ( newval ) : "cc"); }

17. Atomic acquire lock 1 2 3 4 5 6 7 8 9 10 void acquire ( struct spinlock ∗ lk) { pushcli (); // disable interrupts . if( holding (lk)) panic (" acquire "); while (xchg (&lk −>locked , 1) != 0) ; }

18. Atomic release lock 1 2 3 4 5 6 7 8 9 void release ( struct spinlock ∗ lk) { if(! holding (lk)) panic (" release "); xchg (&lk −>locked , 0); popcli (); }

19. Recursive • What happens if a callee tries to acquire a lock held by its caller? • Solution: recursive locks • But they do not ensure mutual exclusion between the caller and the callee • Pass the buck to the programmer

23. Lock usage example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 void iderw ( struct buf ∗ b) { struct buf ∗ ∗ pp; acquire (& idelock ); b −>qnext = 0; for(pp =& idequeue ; ∗ pp; pp =&( ∗ pp)−>qnext) ; ∗ pp = b; // Wait for request to finish . while ((b −>flags & ( B_VALID | B_DIRTY )) != B_VALID ){ sleep (b, & idelock ); } release (& idelock ); }

24. When to use locks • To use: 1 2 A variable can concurrently be written to by multiple CPUs An invariant spans multiple data structures • Possible to protect the kernel through a “giant kernel lock” • Problem(s)? • Reminiscent of the GIL in Python • Possible to have ﬁne-grained locks

31. Lock ordering • If code path x needs locks in the order A and B while y needs them in order B and A, could there be a problem? • Need to ensure that all code paths acquire locks in the same order

32. Lock ordering • If code path x needs locks in the order A and B while y needs them in order B and A, could there be a problem? • Need to ensure that all code paths acquire locks in the same order

33. Interrupt handlers • Locks are also used to synchronize access between interrupt handlers and non-interrupt code

34. Interrupt handlers (2): Example 1 2 3 4 5 6 7 8 9 T_IRQ0 + IRQ_TIMER : if(cpu −>id == 0){ acquire (& tickslock ); ticks ++; wakeup (& ticks ); release (& tickslock ); } lapiceoi (); break ;

35. Interrupt handlers (3): Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 void sys_sleep (void ){ int n; uint ticks0 ; if( argint (0, &n) < 0) return −1; acquire (& tickslock ); ticks0 = ticks; while ( ticks − ticks0 < n){ if(proc −>killed ){ release (& tickslock ); return −1; } sleep (& ticks , & tickslock ); } release (& tickslock );

36. Interrupt handlers • Interrupts lead to concurrency problems on uniprocessors too • Disable interrupts before acquiring a lock: pushcli() • Re-enable on release: popcli() • Avoids recursive locks

40. Today’s Task • xv6 processes only have a single thread so processes do not share any memory • But it is deﬁnitely possible to add multithreading • Imagine that you have been provided a library with methods to create (xv6_thread_create()) and block (xv6_thread_join()) threads • xv6 already provides you with a spinlock with methods to acquire and release that lock. • In addition, it also has methods to: 1 Put a thread to sleep (sleep(void *chan, struct spinlock *lk)) on a variable (*chan) (it releases lk before 2 sleeping and then reacquires it after waking up) Wake up (wakeup(void *chan)) threads sleeping on a variable (*chan)

47. Today’s Task (2) • Design a thread-safe library (in pseudo code) for xv6 that uses these existing primitives, to implement semaphores (binary and counting) and conditional variables and their various methods • Also, add commenting to describe why your solution is thread-safe

48. Reading • Chapter 4 from “xv6: a simple, Unix-like teaching operating system” • Timothy L. Harris. 2001. A Pragmatic Implementation of Non-blocking Linked-Lists. In Proceedings of the 15th International Conference on Distributed Computing (DISC ’01), Jennifer L. Welch (Ed.). Springer-Verlag, London, UK, 300-314. Online: http: //www.timharris.co.uk/papers/2001-disc.pdf

AOS Lab 4: If you liked it, then you should have put a “lock” on it

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AOS Lab 4: If you liked it, then you should have put a “lock” on it