Atomic Instruction

Abstract

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• A read-modify-write (RMW) Instruction on a piece of data, that appears to complete instantaneously and without interruption, even when multiple Thread or processors attempt to access the same piece of data simultaneously
• This ensures that a set of read-modify-write (RMW) operation’s outcome on a piece of data is always consistent and predictable
• Commonly used in Semaphore (信号量) & Mutex (互斥体) etc which have state that need to be accessed and modified safely by multiple Process (进程)

Benefits

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Prevent Race Condition

• With read-modify-write (RMW) united in one pieces, there is no way for one Thread to read, another Thread to write, and first Thread from overwriting the changes made by the second Thread
• Thus, preventing Race Condition (竞态条件)

Examples

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Acquiring and Releasing Spinlock

• Atomic instructions are also used to acquire and release Spinlock (自旋锁), which are locks that prevent multiple threads from accessing a shared resource simultaneously.

// XV6-RISCV Kernel Codes, spinlock.c
 
// Acquire the lock.
// Loops (spins) until the lock is acquired.
void
acquire(struct spinlock *lk)
{
  // ...
 
  // On RISC-V, sync_lock_test_and_set turns into an atomic swap:
  //   a5 = 1
  //   s1 = &lk->locked
  //   amoswap.w.aq a5, a5, (s1)
  while(__sync_lock_test_and_set(&lk->locked, 1) != 0);
 
  // ...
}