Threads in the new ISO C Standard from 2011

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Summary so far An essential property of parallel C programs is that we have ordered all memory accesses through the happens before relation which will be explained in detail soon. If you use mutexes to order the accesses, you will be safe. Atomic objects and fences are intended for use when: Mutexes don’t give sufficient performance. You implement other high level synchronization primitives. We will next explain the happens before relation in detail. Then we will show concrete examples of using atomic objects including implementation details for Power processors. Jonas Skeppstedt (js@cs.lth.se) Lecture 6 2013 26 / 70
Sequenced Before The most common sequence point in C/C++ is semicolon. Others include: Function call Comma operator After evaluating the left operand of ?:, && and ||. Below at L, the assignment to and use of v not sequenced. int u = 1, v = 2, w; L: w = (v = u + 3) + v * 4; It is legal C but the value of w can become either 12 or 20. It is due to it is unspecified which operand of + is evaluated first. In the following w becomes 20 since comma is a sequence point. w = (v = u + 3), v * 4; Jonas Skeppstedt (js@cs.lth.se) Lecture 6 2013 27 / 70
Page 1 and 2: Threads in the new ISO C Standard f
Page 3 and 4: Atomic Objects Atomic objects are n
Page 5 and 6: Using Atomic Objects Members of an
Page 7 and 8: Lock Free Property To know whether
Page 9 and 10: Memory Order for Atomic Operations
Page 11 and 12: Answer: Yes The code may execute in
Page 13 and 14: Atomic Compare and Exchange There a
Page 15 and 16: Weak Compare and Exchange Functions
Page 17 and 18: Atomic Fetch and Modify Functions T
Page 19 and 20: Atomic Flag The atomic flag type an
Page 21 and 22: The C11 Memory Consistency Model fo
Page 23 and 24: Release and Acquire, and Consume Re
Page 25: Happens Before and Data Races We wi
Page 29 and 30: An Example _Atomic int m; int u; //
Page 31 and 32: Dependences Dependences are intra-t
Page 33 and 34: Definition of Dependency in C11 A c
Page 35 and 36: Inter-Thread Happens Before An eval
Page 37 and 38: Read-Copy Update RCU is an alternat
Page 39 and 40: A Simplified Example Consider the f
Page 41 and 42: More Details About Dependences Reca
Page 43 and 44: Constant Propagation Example 2(3) T
Page 45 and 46: One More Data Dependency a = atomic
Page 47 and 48: Load and Reserve/Store Conditional
Page 49 and 50: Power Memory Barrier Instructions T
Page 51 and 52: lwsync Memory Barrier for Release O
Page 53 and 54: c;isync Memory Barrier for Load Acq
Page 55 and 56: Cumulative Ordering If the memory a
Page 57 and 58: Implementation on Power The followi
Page 59 and 60: Store Store relaxed st Store releas
Page 61 and 62: Acquire and Release a Spin Lock # a
Page 63 and 64: Initialization Function void call_o
Page 65 and 66: Mutex Options for mtx_init Function
Page 67 and 68: Condition Variable Functions int cn
Page 69 and 70: Thread Functions int thrd_create(th

Threads in the new ISO C Standard from 2011

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