lecture 7:  interprocess communications

RACE CONDITION
discussion of print spooler (pg 101)

protecting CRITICAL REGIONS with MUTUAL EXCLUSION (MUTEX)

the 4 assumptions:
1.  no two procs may be simultaneously inside their crit regions
2.  no assumptions about speed/# of CPUs.
3.  no proc OUTSIDE its crit region may block others
4.  no proc should have to wait forever to enter its crit region

pg 103 diagram of proc B waiting for proc A to exit its crit region

mutex with BUSY WAITING

-disabling ints?  what if u-proc "forgets" turn back on?  what if
 multi-CPU?  limited OK for kernel.

-lock variables -- requires disabling ints to accomplish.  SPIN LOCK
btw BUSY WAITING == BAD use only if waits expected to be short

-strict alternation (pg 105 code) 
  buggy if one proc is much faster/slower than the other

-peterson's solution

-TSL instruction