Linux-使用互斥锁同步串行端口

Question

I'm writing a C program for Linux OS.
The program can start a timer: both main program and timer can send and receive characters on a serial port.
My attempt is to serialize the serial port access by a mutex in a global structure initialized on the opening with:

if (pthread_mutex_init( &pED->lockSerial, NULL) != 0)
{   
    lwsl_err("lockSerial init failed\n");
}

I protected all the functions that send data on the port as follow:

ssize_t cmdFirmwareVersion(EngineData *pED)
{
  if (pED->fdSerialPort==-1)
    return -1;

  LOCK_SERIAL;
  unsigned char cmd[] = { 0x00, 0x00, 0x7F };
  write( pED->fdSerialPort, cmd, sizeof(cmd));
  int rx = read ( pED->fdSerialPort, rxbuffer, sizeof rxbuffer);
  dump( rxbuffer, rx);
  UNLOCK_SERIAL;
  return rx;
}

where

#define LOCK_SERIAL if (0!=pthread_mutex_lock(&pED->lockSerial)) {printf("Err lock");return 0;}
#define UNLOCK_SERIAL   pthread_mutex_unlock(&pED->lockSerial);

Running the program and starting the timer I see the requests are regular. When I trigger one of this calls on other way (from a rx websocket function) the program hangs and I need to kill it.

Why the entire program stops ??

Answer 1

If a process hangs, it could be because of circular wait for mutexes or holding mutex and trying to lock it again. This could cause deadlock.

ps output will show thread's state as D or S if it's waiting for a resource. it will appear as the process is hung.

               D    uninterruptible sleep (usually IO)
               S    interruptible sleep (waiting for an event to complete)

I have made a thread to hold mutex and try to lock it again. ps output and GDB shows main thread and child thread are in sleep.

xxxx@virtualBox:~$ ps -eflT |grep a.out
0 S root      3982  3982  2265  0  80   0 - 22155 -      20:28 pts/0    00:00:00 ./a.out
1 S root      3982  3984  2265  0  80   0 - 22155 -      20:28 pts/0    00:00:00 ./a.out

(gdb) info threads 
  Id   Target Id         Frame 
* 1    Thread 0x7ffff7fdf740 (LWP 4625) "a.out" 0x00007ffff7bbed2d in     __GI___pthread_timedjoin_ex (
    threadid=140737345505024, thread_return=0x0, abstime=0x0, block=    <optimized out>) at pthread_join_common.c:89
  2    Thread 0x7ffff77c4700 (LWP 4629) "a.out" __lll_lock_wait ()
    at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135

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