Question

So I had some shared_mutex and done this:

        boost::upgrade_lock<boost::shared_mutex> lock(f->mutex);
        boost::upgrade_to_unique_lock<boost::shared_mutex> uniqueLock(lock);

now I want to “unlock it” or at least downgrade it to something like:

boost::shared_lock<boost::shared_mutex> lock_r(f->mutex);

How to do such thing? Is it possible?

Answer 1

If you let the upgrade_to_unique_lock go out of scope, it will automatically downgrade back to upgrade ownership.

For example

void foo() {
   boost::upgrade_lock<boost::shared_mutex> lock(f->mutex);

   // Do shared operations, as mutex is held upgradeable
   // ...

   if(need_to_get_unique)
   {
      boost::upgrade_to_unique_lock<boost::shared_mutex> uniqueLock(lock); 

      // Do exclusive operations, as mutex is held uniquely
      // ... 
      // At end of scope unique is released back to upgradeable
   }
   // Only shared operations here, as it's only held upgradeable
   // ...

   // At end of scope mutex is completely released
}

Edit: One other thing. If a given function only requires exclusive locks, you can use boost::unique_lock and lock uniquely, without going through both the upgrade and upgrade_to_unique locks.