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lab7 理解race condition
x86模拟运行环境
x86.py是一个模拟执行基于汇编代码的多线程执行过程的模拟器。这里的汇编语法是基于很简单的x86汇编语法。 且没有包括OS的调度、context切换和中断处理过程。每条指令大小为一个byte。每个变量占4个byte 在硬件上模拟了4个通用寄存器：
%ax, %bx, %cx, %dx
一个程序计数器pc，一个堆栈寄存器sp，和一小部分指令：
mov immediate, register # immediate value --> register
mov memory, register # memory --> register
mov register, register # register --> register
mov register, memory # register --> memory
mov immediate, memory # immediate value --> memory
add immediate, register # register = register + immediate
add register1, register2 # register2 = register2 + register1
sub immediate, register # register = register - immediate
sub register1, register2 # register2 = register2 - register1
test immediate, register # compare immediate and register (set condition codes)
test register, immediate # compare register and immediate (set condition codes)
test register, register # compare register and register (set condition codes)
jne # jump if test'd values are not equal
je # jump if test'd values are equal
jlt # jump if test'd second is less than first
jlte # jump if test'd second is less than or equal first
jgt # jump if test'd second is greater than first
jgte # jump if test'd second is greater than or equal first
xchg register, memory # atomic exchange:
# put value of register into memory
# return old contents of memory into reg
# do both things atomically
nop # no op
halt # stop
push memory or register # push value in memory or from reg onto stack
# stack is defined by sp register
pop [register] # pop value off stack (into optional register)
call label # call function at label
yield # switch to the next thread in the runqueue
注意:
- 'immediate' 格式是 $number
- 'memory' 格式是 'number' 或 '(reg)' 或 'number(reg)' 或 'number(reg,reg)'
- (%cx) -> 在括号中的register cx 的值 形成 address
- 2000 -> 2000 形成 address
- 1000(%dx) -> 1000 + dx的值 形成 address
- 10(%ax,%bx) -> 10 + ax的值 + bx的值 形成 address
- 'register' 格式是 %ax, %bx, %cx, %dx
下面是一个代码片段：
.main
mov 2000, %ax # 取地址2000处的内存单元的内容，并赋值给ax
add $1, %ax # ax=ax+1
mov %ax, 2000 # 把ax的内容存储到地址2000处的内存单元中
halt
其含义如下
2000 -> 2000 形成地址 address
(%cx) -> cx的内容 形成地址 address
1000(%dx) -> （1000+dx） 形成地址 address
10(%ax,%bx) -> (10+ax+bx) 形成地址 address
halt -> 执行结束
循环执行的小例子片段
.main
.top
sub $1,%dx
test $0,%dx
jgte .top
halt
x86.py模拟器运行参数
-h, --help show this help message and exit
-s SEED, --seed=SEED the random seed
-t NUMTHREADS, --threads=NUMTHREADS
number of threads
-p PROGFILE, --program=PROGFILE
source program (in .s)
-i INTFREQ, --interrupt=INTFREQ
interrupt frequency
-r, --randints if interrupts are random
-a ARGV, --argv=ARGV comma-separated per-thread args (e.g., ax=1,ax=2 sets
thread 0 ax reg to 1 and thread 1 ax reg to 2);
specify multiple regs per thread via colon-separated
list (e.g., ax=1:bx=2,cx=3 sets thread 0 ax and bx and
just cx for thread 1)
-L LOADADDR, --loadaddr=LOADADDR
address where to load code
-m MEMSIZE, --memsize=MEMSIZE
size of address space (KB)
-M MEMTRACE, --memtrace=MEMTRACE
comma-separated list of addrs to trace (e.g.,
20000,20001)
-R REGTRACE, --regtrace=REGTRACE
comma-separated list of regs to trace (e.g.,
ax,bx,cx,dx)
-C, --cctrace should we trace condition codes
-S, --printstats print some extra stats
-v, --verbose print some extra info
-c, --compute compute answers for me
执行举例
$ ./x86.py -p simple-race.s -t 1 -M 2000 -R ax,bx
2000 ax bx Thread 0
? ? ?
? ? ? 1000 mov 2000, %ax
? ? ? 1001 add $1, %ax
? ? ? 1002 mov %ax, 2000
? ? ? 1003 halt
如果加上参数 -c可得到具体执行结果
$ ./x86.py -p simple-race.s -t 1 -M 2000 -R ax,bx -c
2000 ax bx Thread 0
0 0 0
0 0 0 1000 mov 2000, %ax
0 1 0 1001 add $1, %ax
1 1 0 1002 mov %ax, 2000
1 1 0 1003 halt
另外一个执行的例子
$ ./x86.py -p loop.s -t 1 -a dx=3 -R dx -C -c
dx >= > <= < != == Thread 0
3 0 0 0 0 0 0
2 0 0 0 0 0 0 1000 sub $1,%dx
2 1 1 0 0 1 0 1001 test $0,%dx
2 1 1 0 0 1 0 1002 jgte .top
1 1 1 0 0 1 0 1000 sub $1,%dx
1 1 1 0 0 1 0 1001 test $0,%dx
1 1 1 0 0 1 0 1002 jgte .top
0 1 1 0 0 1 0 1000 sub $1,%dx
0 1 0 1 0 0 1 1001 test $0,%dx
0 1 0 1 0 0 1 1002 jgte .top
0 1 0 1 0 0 1 1003 halt
多线程存在race condition 的例子 looping-race-nolock.s
.main
.top
# critical section
mov 2000, %ax # get the value at the address
add $1, %ax # increment it
mov %ax, 2000 # store it back
# see if we're still looping
sub $1, %bx
test $0, %bx
jgt .top
halt
执行结果：
$ ./x86.py -p looping-race-nolock.s -t 2 -a bx=1 -M 2000 -c
2000 bx Thread 0 Thread 1
0 1
0 1 1000 mov 2000, %ax
0 1 1001 add $1, %ax
1 1 1002 mov %ax, 2000
1 0 1003 sub $1, %bx
1 0 1004 test $0, %bx
1 0 1005 jgt .top
1 0 1006 halt
1 1 ----- Halt;Switch ----- ----- Halt;Switch -----
1 1 1000 mov 2000, %ax
1 1 1001 add $1, %ax
2 1 1002 mov %ax, 2000
2 0 1003 sub $1, %bx
2 0 1004 test $0, %bx
2 0 1005 jgt .top
2 0 1006 halt
多线程存在 race condition 的例子 looping-race-nolock.s 在引入中断后，会产生race condition.
$ ./x86.py -p looping-race-nolock.s -t 2 -a bx=1 -M 2000 -i 2
2000 Thread 0 Thread 1
?
? 1000 mov 2000, %ax
? 1001 add $1, %ax
? ------ Interrupt ------ ------ Interrupt ------
? 1000 mov 2000, %ax
? 1001 add $1, %ax
? ------ Interrupt ------ ------ Interrupt ------
? 1002 mov %ax, 2000
? 1003 sub $1, %bx
? ------ Interrupt ------ ------ Interrupt ------
? 1002 mov %ax, 2000
? 1003 sub $1, %bx
? ------ Interrupt ------ ------ Interrupt ------
? 1004 test $0, %bx
? 1005 jgt .top
? ------ Interrupt ------ ------ Interrupt ------
? 1004 test $0, %bx
? 1005 jgt .top
? ------ Interrupt ------ ------ Interrupt ------
? 1006 halt
? ----- Halt;Switch ----- ----- Halt;Switch -----
? 1006 halt