【OS复习】Syncronization

最新推荐文章于 2022-03-27 21:32:21 发布

原创最新推荐文章于 2022-03-27 21:32:21 发布 · 511 阅读

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#os #semaphore #access

本文深入探讨了并发控制机制，包括临界区、互斥、测试并设置指令、比较并交换指令等基本概念，并介绍了信号量的忙等待及阻塞实现方式，以及读写锁的工作原理。

Critical Section
- mutual exclusion: only one has access
- forward progress: if no one is accessing, then can access
- bounded waiting: eventually will access, not wait for ever

n threads - 1 CPU

n threads, n CPUs (every CPU has its own intr line; need hardware support for SMP (atomic operations))
- test_and_set: ts(x): if 0, set 1. return x.

- compare and swap: if !eq swap

lock(x) { key = 1; while (key){cs(x, key);} } unlock(x) { x=0; }

Semaphore: Busy Waiting
- 1 CPU

P(s) { disable_intr(); while(s<=0){ enable_intr(); disable_intr(); } s--; enable_intr(); } V(s) { disable_intr(); s++; enable_intr(); }

- n CPUs

P(s) { while(ts(s)); } V(s) { s = 0; }

Semaphore: Blocking

P(s) { s--; if(s<0){ add process to wait queue; block; } } V(s) { s++; if(s<=0){ remove process p from a wait queue; wakeup(p); } }

Read Write Lock

/* readers: */ lockshared() { lock(x) while(count<0)} unlock(x); lock(x); } count++; unlock(x); } unlockshared() { lock(x); count--; unlock(x); } /* writers: */ lockexclusive() { lock(x); while(count){ unlock(x); lock(x); } count = -1; unlock(x); } unlockexclusive() { lock(x); count = 0; unlock(x); }

Or:

lockshared() { lock(x); readcount++; if(readcount>-1)lock(y); unlock(x) } unlockshared() { lock(x); readcount--; if (readcount == 0) unlock(y); unlock(x); } lockexclusive() { lock(y); } unlockexclusive() { unlock(y); }