Threads in the new ISO C Standard from 2011

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A More Detailed Look int b; void first(void) { list_t* q = rcu_dereference(head); } return q->a + b; Before the proposal for dependency ordering through release/consume, the then current draft would require the use of an acquire operation in the rcu_dereference. Doing so prevents the compiler from loading b before the execution of the rcu_dereference. A standard for a high performance language must permit extensive compiler optimization and efficient execution on modern machines. C11 succeeds with this — also for multicores. Jonas Skeppstedt (js@cs.lth.se) Lecture 6 2013 40 / 70
More Details About Dependences Recall the intra-thread A carries a dependency to B. The compiler becomes responsible for not optimizing away such dependences. This might sound trivial but is not. See below. A dependency is started with a consume operation: p = atomic_load(q, memory_order_consume); a = *p + 1; A root of a dependency tree is created with the consume, and the memory read in *p becomes ordered, as we expect. The compiler or hardware is not allowed to move the *p to before the first line which would probably not make much sense, anyway. Behaviour is as we want and expect. Jonas Skeppstedt (js@cs.lth.se) Lecture 6 2013 41 / 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 and 26: Happens Before and Data Races We wi
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: A Simplified Example Consider the f
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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