Threads in the new ISO C Standard from 2011

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Conflicting Expression Evaluations Two expression evaluations conflict if they access the same memory location and at least one of them modifies that location. For example: // Thread 1 // Thread 2 a = b + c; d = a + 1; An assignment is an expression so the code above conflict. By evaluation is meant that the expressions are computed at runtime. Conflicting evaluations are necessary in parallel programs unless each thread can work exclusively on its own data. Conflicting evaluations become a big problem if they are not ordered through the happens before relation. Jonas Skeppstedt (js@cs.lth.se) Lecture 6 2013 24 / 70
Happens Before and Data Races We will look at the details of the happens before relation and what it means in terms of executing C code at the machine instruction level below, after introducing happens before informally. It is the unlock/lock pair which guarantees that the write happens before the read and that the data becomes visible to the other thread. Since there is no synchronization in the previous slide there is a data race, obviously. An unlock on m synchronizes with a lock on m. Recall that mutex unlock/lock is one of three main kinds of synchronizations which orders two variable accesses. The other two are: Release on an atomic object M followed by acquire or consume on M Memory fences. There are additional important details for atomic objects and fences which we will see later. Jonas Skeppstedt (js@cs.lth.se) Lecture 6 2013 25 / 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: Release and Acquire, and Consume Re
Page 27 and 28: Sequenced Before The most common se
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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