MIT 6.S081 Lecture 10: Multiprocessors and locking

Reading

• Read Locking
• read code
  

Locks introduction

• apps wants to multiple cores

• kernel must handle parallel sys calls

• access shared data structure in parallel

• -> locks for correct sharing

• locks limit performance

Microprocessor trend

Lock abstraction

• struct lock

// Mutual exclusion lock.
struct spinlock {
  uint locked;       // Is the lock held?

  // For debugging:
  char *name;        // Name of lock.
  struct cpu *cpu;   // The cpu holding the lock.
};

acquire(&l)
// critical section
// your code here
release(&l)

• 程序有很多把锁，如果只有一把，串行影响性能，为达到某种程度的并行且保证正确性，程序使用多把锁

When to lock?

• Constructive Rule: two process access a shared data structure + one is writing

• too strict: lock free programming

• too loose: printf(“xxxx”)

Could locking be automatic?

Lock perspective

• avoid lost update

• make multi-step operation atomatic

• help maintain invariant

Dead lock

Locks vs modual

• lock ordering -> global

• 锁使模块化变得困难

Locks vs performance

Implenment Lock

memory ordering

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