How many simultaneous readers can a pthread_rwlock have?

Question

I have a multi-threaded application that creates 48 threads that all need to access a common attribute (stl::map). The map will only be written to when the threads start, and the rest of the time the map will be read from. This seems like the perfect use-case for a pthread_rw_lock, and all appears to be working well.

I ran across a completely unrelated seg-fault and started analyzing the core. Using gdb, I executed the command info threads and was quite surprised at the results. I observed that several threads were actually reading from the map as expected, but the strange part is that several threads were blocked in pthread_rwlock_rdlock() waiting on the rw_lock.

Here is the stack trace for a thread that is waiting on the lock:

#0  0xffffe430 in __kernel_vsyscall ()
#1  0xf76fe159 in __lll_lock_wait () from /lib/libpthread.so.0
#2  0xf76fab5d in pthread_rwlock_rdlock () from /lib/libpthread.so.0
#3  0x0804a81a in DiameterServiceSingleton::getDiameterService(void*) ()

With so many threads, its difficult to say how many were reading and how many were blocked, but I dont understand why any threads would be blocked waiting to read, considering other threads are already reading.

So here is my question: Why are some threads blocked waiting to read a rw_lock, when other threads are already reading from it? It appears as though there is a limit to the number of threads that can simultaneously read.

Ive looked at the pthread_rwlock_attr_t functions and didnt see anything related.

The OS is Linux, SUSE 11.

Here is the related code:

{
  pthread_rwlock_init(&serviceMapRwLock_, NULL);
}

// This method is called for each request processed by the threads
Service *ServiceSingleton::getService(void *serviceId)
{
  pthread_rwlock_rdlock(&serviceMapRwLock_);
    ServiceMapType::const_iterator iter = serviceMap_.find(serviceId);
    bool notFound(iter == serviceMap_.end());
  pthread_rwlock_unlock(&serviceMapRwLock_);

  if(notFound)
  {
    return NULL;
  }

  return iter->second;
}

// This method is only called when the app is starting
void ServiceSingleton::addService(void *serviceId, Service *service)
{
  pthread_rwlock_wrlock(&serviceMapRwLock_);
    serviceMap_[serviceId] = service;
  pthread_rwlock_unlock(&serviceMapRwLock_);
}

Update:

As mentioned in the comments by MarkB, if I had set pthread_rwlockattr_getkind_np() to give priority to writers, and there is a writer blocked waiting, then the observed behavior would make sense. But, Im using the default value which I believe is to give priority to readers. I just verified that there are no threads blocked waiting to write. I also update the code as suggested by @Shahbaz in the comments and get the same results.

How many simultaneous readers can a pthread_rwlock have?

2 Answers