pthread mutex locking variables used in statements

问题

First of all, I have a gut feeling that says, inside an if statement, if I am using the variable, it counts as reading the variable so I should lock it with mutex there also (if another pthread might be doing stuff with it). Am I right that I should lock it?

The example situation in simplified manner is given below.

in one thread I am using the below statement:

if(event){
  // Should I or should I not lock event here to use it
  // inside if statement?
  pthread_mutex_lock(&mutex_event);
  event = 0;
  pthread_mutex_unlock(&mutex_event);
  // blah blah code here
  // blah blah code here
  // blah blah code here
}

in another thread I am doing

pthread_mutex_lock(&mutex_event);
event = 1;
pthread_mutex_unlock(&mutex_event);

Second question, if I do need to lock it, how should I do it in a classy programmers manner? In other words, what is the general convention. I don't like the below idea as I have to wait for all of the lines inside "if" to execute to be able to unlock mutex again.

pthread_mutex_lock(&mutex_event);
if(event){ 
  event = 0;
  // blah blah code here
  // blah blah code here
  // blah blah code here
}
pthread_mutex_unlock(&mutex_event);

I am ok with this idea, but probably it could look prettier:

pthread_mutex_lock(&mutex_event);
if(event){    
  event = 0;
  pthread_mutex_unlock(&mutex_event);
  // blah blah code here
  // blah blah code here
  // blah blah code here
}
else
{
  pthread_mutex_unlock(&mutex_event);
}

I find that it gets trickier with a while loop and this is the primitive solution I have got:

pthread_mutex_lock(&mutex_event);
store_event = event; // store_event is local
pthread_mutex_unlock(&mutex_event);
while(store_event){ 
  // blah blah code here
  // blah blah code here
  // blah blah code here
}

回答1:

Every access to the shared variable - write AND read - should be guarded. How much else you surround by mutexes is an individual matter - balancing the overhead, atomicity of using your variable and clarity of code.

Also, you don't want to spend a long time in the protected area, so if you synchronize a variable two times in a long section of code, you don't want to lock the whole big section restricted.

There's obviously a problem with some reads:

pthread_mutex_lock(&mutex_event);
if(event){ 
  event = 0;
  pthread_mutex_unlock(&mutex_event);
  // blah blah code here
  // blah blah code here
  // blah blah code here
}
else pthread_mutex_unlock(&mutex_event); //awful, a hundred lines below the opening.

It's much better in that case to leave distinct 'synchronization zones', and operate on a local copy of the variable.

pthread_mutex_lock(&mutex_event);
event_copy = event;
data_copy = data;
state_copy = state;
pthread_mutex_unlock(&mutex_event);

if(event_copy){ 
  // blah blah code here
  // blah blah code here
  event_copy = 0;
  // blah blah code here
}
// blah blah code here

pthread_mutex_lock(&mutex_event);
event = event_copy;
data = data_copy;
state = state_copy;
pthread_mutex_unlock(&mutex_event);

and so on.

That way frequent uses to given variable don't require the wards, there's no risk of forgetting some unlock (like lack of that 'else' statement!) and you bundle work minimizing time spent waiting in mutexes or locking/unlocking them.

Also remember: not to lose the synchronization data to cache, all inter-thread variables should be declared as volatile. Otherwise it may take a long time before your event propagates from one thread to another. But using volatile you break a lot of optimizations by the compiler. By making the non-volatile local copies you reduce the amount of work done around the volatile variables and allow the optimizer to go wild all over the copies without risk of breaking things.

回答2:

Yes, you do need synchronisation for all reads.

For the while() loop you can use this pattern:

pthread_mutex_lock(&mutex_event);
while(event) {
  pthread_mutex_unlock(&mutex_event);
  // blah blah code here
  // blah blah code here
  // blah blah code here
  pthread_mutex_lock(&mutex_event);
}
pthread_mutex_unlock(&mutex_event);

..So the lock is always held at the condition.

回答3:

Yes, you should always lock the mutex before reading or writing the protected variable. To avoid messy code, while still minimizing lock time, you should move the event information to a local variable that can be used after the mutex is unlocked, like this

int do_blah = 0;

pthread_mutex_lock(&mutex_event);
if(event){
    event = 0;
    do_blah = 1;
}
pthread_mutex_unlock(&mutex_event);

if ( do_blah ) {
    // blah blah code here
    // blah blah code here
    // blah blah code here
}

来源：https://stackoverflow.com/questions/37133903/pthread-mutex-locking-variables-used-in-statements

标签

Linux

pthreads

posix

mutex