Multicore & Synchronization
The two threads above run simultaneously on different cores (assume no compiler reordering). Which outcome cannot occur under sequential consistency but can occur on real CPUs such as x86?
// Initially x = 0, y = 0
// Thread 1 // Thread 2
x = 1; y = 1;
r1 = y; r2 = x;- Ar1 = 1 and r2 = 1
- Br1 = 0 and r2 = 1
- Cr1 = 0 and r2 = 0
Show answer and explanation
Answer: C. r1 = 0 and r2 = 0
Under sequential consistency one of the writes happens first, so at least one read must see 1. On real CPUs, however, x = 1 first enters the core's store buffer, and before it becomes visible to other cores the later read r1 = y can fetch its value from cache or memory. If both cores do this, both read 0. A memory barrier between the write and the read prevents it.
- A. This occurs under sequential consistency if both writes finish first.
- B. This occurs under sequential consistency if thread 1 finishes first.
- C. Correct. Reads proceed while the writes sit in store buffers.