Outline. Deadlock. Examples, Conditions, Strategies. Deadlock Prevention. Deadlock Avoidance. Banker s algorithm

Transcription

1 Deadlock

2 Outline Deadlock Examples, Conditions, Strategies Deadlock Prevention Deadlock Avoidance Banker s algorithm

3 Deadlocks Thread 1: lock(l1); lock(l2);! Thread 2: lock(l2); lock(l1);

4 Examples?

5 Examples? GFP_IO flag in Linux Kernel Memory allocator Causes get_free_page() calls shm_swap() to pageout. What if shm_swap() needs memory?

6 Examples? GFP_IO flag in Linux Kernel Memory allocator Causes get_free_page() calls shm_swap() to pageout. What if shm_swap() needs memory? Vector v1, v2; v1.addall(v2); addall() needs to be thread safe. what happens when v2.addall(v1) is called too?

7 Conditions

8 Conditions Mutual Exclusion threads claim exclusive control

9 Conditions Mutual Exclusion threads claim exclusive control Hold-and-Wait resources acquired progressively

10 Conditions Mutual Exclusion threads claim exclusive control Hold-and-Wait resources acquired progressively No preemption resources cannot be forcibly taken away

11 Conditions Mutual Exclusion threads claim exclusive control Hold-and-Wait resources acquired progressively No preemption resources cannot be forcibly taken away Circular Wait circular chain such that threads wait on resources held by another

12 Dining Philosophers

13 Dining Philosophers Philosopher has sole control over picked fork (mutual exclusion)

14 Dining Philosophers Philosopher has sole control over picked fork (mutual exclusion) Philosopher picks left fork then picks right (hold and wait)

15 Dining Philosophers Philosopher has sole control over picked fork (mutual exclusion) Philosopher picks left fork then picks right (hold and wait) Fork cannot be taken away from a philosopher (no preemption)

16 Dining Philosophers Philosopher has sole control over picked fork (mutual exclusion) Philosopher picks left fork then picks right (hold and wait) Fork cannot be taken away from a philosopher (no preemption) Once all philosophers pick a fork, each of them wait on the philosopher at his right to release the fork (circular wait)

17 Strategies

18 Ignore - Ostrich algorithm Strategies

19 Strategies Ignore - Ostrich algorithm Detection & Recovery

20 Strategies Ignore - Ostrich algorithm Detection & Recovery Build Dependency graph

21 Strategies Ignore - Ostrich algorithm Detection & Recovery Build Dependency graph

22 Strategies Ignore - Ostrich algorithm Detection & Recovery Build Dependency graph

23 Strategies Ignore - Ostrich algorithm Detection & Recovery Build Dependency graph Identify deadlocked processes

24 Strategies Ignore - Ostrich algorithm Detection & Recovery Build Dependency graph Identify deadlocked processes rollback, preempt, kill

25 Strategies Ignore - Ostrich algorithm Detection & Recovery Build Dependency graph Identify deadlocked processes rollback, preempt, kill Prevention

26 Strategies Ignore - Ostrich algorithm Detection & Recovery Build Dependency graph Identify deadlocked processes rollback, preempt, kill Prevention Negate any one of the deadlock conditions

27 Strategies Ignore - Ostrich algorithm Detection & Recovery Build Dependency graph Identify deadlocked processes rollback, preempt, kill Prevention Negate any one of the deadlock conditions Avoidance

28 Strategies Ignore - Ostrich algorithm Detection & Recovery Build Dependency graph Identify deadlocked processes rollback, preempt, kill Prevention Negate any one of the deadlock conditions Avoidance Carefully allocate resources

29 Prevention - Mutual Exclusion Never assign resource exclusively to a process Not really realistic. ex. printers can t print two documents simultaneously Use daemon to exclusively access printer? Deadlock free? Use hardware instructions, compare and swap

30 Prevention - Hold & Wait down(global_sem); down(get_left(i)); down(get_right(i)); up(global_sem);! -! down(global_sem); up(get_left(i)); up(get_right(i)); up(global_sem);

31 Prevention - Hold & Wait Ensure that all resources are acquired at once down(global_sem); down(get_left(i)); down(get_right(i)); up(global_sem);! -! down(global_sem); up(get_left(i)); up(get_right(i)); up(global_sem);

32 Prevention - Hold & Wait Ensure that all resources are acquired at once Is this feasible? down(global_sem); down(get_left(i)); down(get_right(i)); up(global_sem);! -! down(global_sem); up(get_left(i)); up(get_right(i)); up(global_sem);

33 Prevention - Hold & Wait Ensure that all resources are acquired at once Is this feasible? Know all resources in advance? down(global_sem); down(get_left(i)); down(get_right(i)); up(global_sem);! -! down(global_sem); up(get_left(i)); up(get_right(i)); up(global_sem);

34 Prevention - No Preemption

35 Prevention - No Preemption Take away resources owned by thread

36 Prevention - No Preemption Take away resources owned by thread Use trylock(), non blocking call in getting the lock.

37 Prevention - No Preemption Take away resources owned by thread Use trylock(), non blocking call in getting the lock. top: lock(get_left(i); if (trylock(get_right(i) == -1) { unlock(get_left(i)); goto top; }

38 Prevention - No Preemption Take away resources owned by thread Use trylock(), non blocking call in getting the lock. top: lock(get_left(i); if (trylock(get_right(i) == -1) { unlock(get_left(i)); goto top; } Lifelock?

39 Prevention - Circular Wait

40 Prevention - Circular Wait Enforce ordering in Locking

41 Prevention - Circular Wait Enforce ordering in Locking enumerate all locks in the system?

42 Prevention - Circular Wait Enforce ordering in Locking enumerate all locks in the system? 62 /* 63 * Lock ordering: 64 * 65 * ->i_mmap_rwsem (truncate_pagecache) 66 * ->private_lock ( free_pte-> set_page_dirty_buffers) 67 * ->swap_lock (exclusive_swap_page, others) 68 * ->mapping->tree_lock 69 * 70 * ->i_mutex 71 * ->i_mmap_rwsem (truncate->unmap_mapping_range) 72 * 73 * ->mmap_sen 74 * ->i_mmap_rwsem 75 * ->page_table_lock or pte_lock (various, mainly in memory.c) 76 * ->mapping->tree_lock (arch-dependent flush_dcache_mmap_lock) 77 * 78 * ->mmap_sem 79 * ->lock_page (access_process_vm) 80 * 81 * ->i_mutex (generic_perform_write) 82 * ->mmap_sem (fault_in_pages_readable->do_page_fault) 83 * 84 * bdi->wb.list_lock 85 * sb_lock (fs/fs-writeback.c) 86 * ->mapping->tree_lock ( sync_single_inode) 87 * 88 * ->i_mmap_rwsem 89 * ->anon_vma.lock (vma_adjust) 90 * 91 * ->anon_vma.lock 92 * ->page_table_lock or pte_lock (anon_vma_prepare and various) 93 * 94 * ->page_table_lock or pte_lock 95 * ->swap_lock (try_to_unmap_one)

43 Prevention - Circular Wait Enforce ordering in Locking enumerate all locks in the system? Is it feasible? 62 /* 63 * Lock ordering: 64 * 65 * ->i_mmap_rwsem (truncate_pagecache) 66 * ->private_lock ( free_pte-> set_page_dirty_buffers) 67 * ->swap_lock (exclusive_swap_page, others) 68 * ->mapping->tree_lock 69 * 70 * ->i_mutex 71 * ->i_mmap_rwsem (truncate->unmap_mapping_range) 72 * 73 * ->mmap_sen 74 * ->i_mmap_rwsem 75 * ->page_table_lock or pte_lock (various, mainly in memory.c) 76 * ->mapping->tree_lock (arch-dependent flush_dcache_mmap_lock) 77 * 78 * ->mmap_sem 79 * ->lock_page (access_process_vm) 80 * 81 * ->i_mutex (generic_perform_write) 82 * ->mmap_sem (fault_in_pages_readable->do_page_fault) 83 * 84 * bdi->wb.list_lock 85 * sb_lock (fs/fs-writeback.c) 86 * ->mapping->tree_lock ( sync_single_inode) 87 * 88 * ->i_mmap_rwsem 89 * ->anon_vma.lock (vma_adjust) 90 * 91 * ->anon_vma.lock 92 * ->page_table_lock or pte_lock (anon_vma_prepare and various) 93 * 94 * ->page_table_lock or pte_lock 95 * ->swap_lock (try_to_unmap_one)

44 Deadlock Avoidance Banker s algorithm - Dijkstra (1965) Safe and unsafe states Checks to see if any request would yield a safe state If not, the request is denied

45 Banker s algorithm

46 Banker s algorithm Using Vector E (Existing Resources) and P (Possessed Resources) build

47 Banker s algorithm Using Vector E (Existing Resources) and P (Possessed Resources) build A: Available resources

48 Banker s algorithm Using Vector E (Existing Resources) and P (Possessed Resources) build A: Available resources Pick a row whose resource needs are smaller or equal to A. If not found, system would eventually deadlock as resources cannot be found.

49 Banker s algorithm Using Vector E (Existing Resources) and P (Possessed Resources) build A: Available resources Pick a row whose resource needs are smaller or equal to A. If not found, system would eventually deadlock as resources cannot be found. Assuming the process makes a maximum allocation of its resources, consider the process as terminated and add its resources to the A vector

50 Banker s algorithm Using Vector E (Existing Resources) and P (Possessed Resources) build A: Available resources Pick a row whose resource needs are smaller or equal to A. If not found, system would eventually deadlock as resources cannot be found. Assuming the process makes a maximum allocation of its resources, consider the process as terminated and add its resources to the A vector Repeat until all processes are terminated.

51 Problem #1 Is the State Safe? E = (6, 3, 4, 2) Allocated: A B C D P P P P P Needed: A B C D P P P P P

52 Problem #2 Is the state safe? E = (10, 5, 7) Allocated: A B C P P P P P Max: A B C P P P P P

53 Problem #3 Is the following state safe? E = (6, 5, 7, 6) Allocated: A B C D Max: A B C D P P P P P P

54 Done!!!