Linux GDB分析死锁

目录

示例代码

分析

结论

---

示例代码

// DeadLock.cpp : 定义控制台应用程序的入口点。
//

#include 
#include 
#include 
#include 

static std::mutex mutex1;
static std::mutex mutex2;

static int threadFunc1()
{		
	std::cout << "Enter threadFunc1" << std::endl;
	std::lock_guard lock1(mutex1);	
	std::this_thread::sleep_for(std::chrono::seconds(20));
	std::lock_guard lock2(mutex2);
	std::cout << "Leave threadFunc1" << std::endl;	
	return 0;
}

static int threadFunc2()
{
	std::cout << "Enter threadFunc2" << std::endl;
	std::lock_guard lock1(mutex2);		
	std::this_thread::sleep_for(std::chrono::seconds(20));	
	std::lock_guard lock2(mutex1);
	std::cout << "Leave threadFunc2" << std::endl;	
	return 0;
}

int main()
{
	std::thread thread1(threadFunc1);
	std::thread thread2(threadFunc2);
	
	thread1.join();
	thread2.join();

	std::cout << "All thread end,now exit" << std::endl;	
    return 0;
}

编译运行（目标程序为testDemo）：./testDemo

Enter threadFunc1 Enter threadFunc2

并未输出：Leave threadFunc1 、 Leave threadFunc2、All thread end,now exit，随后程序处于挂起状态

分析

1、Gdb调试：gdb ./testDemo

2、 待到程序无反应时，通过Ctrl+C中断程序运行

3、查看线程信息：info threads

Id   Target Id         Frame * 1    Thread 0x7ffff7fdb740 (LWP 4315) "testDemo" 0x00007ffff7bbed2d in __GI___pthread_timedjoin_ex (threadid=140737335809792, thread_return=0x0,     abstime=0x0, block=) at pthread_join_common.c:89   2    Thread 0x7ffff6e85700 (LWP 4319) "testDemo" __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135   3    Thread 0x7ffff6684700 (LWP 4320) "testDemo" __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135

我们看到，一共有三个线程，分别为主线程，以及两个子线程，线程号分别为4319及4320。并且两个子线程都处于锁等待的状态。

4、查看子线程4319堆栈信息：thread 2; bt（2 是相对线程号）

#0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135 #1  0x00007ffff7bc0023 in __GI___pthread_mutex_lock (mutex=0x5555557581a0 ) at ../nptl/pthread_mutex_lock.c:78 #2  0x00005555555550bf in __gthread_mutex_lock (__mutex=0x5555557581a0 ) at /usr/include/x86_64-linux-gnu/c++/7/bits/gthr-default.h:748 #3  0x0000555555555614 in std::mutex::lock (this=0x5555557581a0 ) at /usr/include/c++/7/bits/std_mutex.h:103 #4  0x0000555555555690 in std::lock_guard::lock_guard (this=0x7ffff6e84dc0, __m=...) at /usr/include/c++/7/bits/std_mutex.h:162 #5  0x0000555555555183 in threadFunc1 () at DeadLock.cpp:17 省略…..

子线程4319在等待mutex2

5、查看子线程4320堆栈信息：thread 3; bt（3 是相对线程号）

#0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135 #1  0x00007ffff7bc0023 in __GI___pthread_mutex_lock (mutex=0x555555758160 ) at ../nptl/pthread_mutex_lock.c:78 #2  0x00005555555550bf in __gthread_mutex_lock (__mutex=0x555555758160 ) at /usr/include/x86_64-linux-gnu/c++/7/bits/gthr-default.h:748 #3  0x0000555555555614 in std::mutex::lock (this=0x555555758160 ) at /usr/include/c++/7/bits/std_mutex.h:103 #4  0x0000555555555690 in std::lock_guard::lock_guard (this=0x7ffff6683dc0, __m=...) at /usr/include/c++/7/bits/std_mutex.h:162 #5  0x000055555555529e in threadFunc2 () at DeadLock.cpp:27

子线程4320在等待锁mutex1

6、查看mutex1、mutex2的信息

p mutex1：

{   = {     _M_mutex = pthread_mutex_t = {       Type = Normal,       Status = Acquired, possibly with waiters,       Owner ID = 4319,       Robust = No,       Shared = No,       Protocol = None     }   }

p mutex2：

{   = {     _M_mutex = pthread_mutex_t = {       Type = Normal,       Status = Acquired, possibly with waiters,       Owner ID = 4320,       Robust = No,       Shared = No,       Protocol = None     }   }

mutex1的当前所属线程是4319（相对线程号为2），mutex2的当前所属线程是4320（相对线程号为3）。同时，mutex1及mutex2中的Status字段均表示其已经被获取，并且可能有等待者。

结论

将上述分析结果以表格形式汇总：

相对线程号	线程号	拥有锁	等待锁
2	4319	mutex1	mutex2
3	4320	mutex2	mutex1

从汇总结果看，线程4319,43202形成了死锁，导致程序挂起。

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