【OS复习】Syncronization

 

• 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

lock(x) { disable_intr(); } unlock(x) { enable_intr(); } 

• 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.

lock(x) { while(ts(x)); } unlock(x) { x=0; } 

•  
  • 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); }