[boost] Lock unification (shared vs. upgradable)

From: Bronek Kozicki (brok_at_[hidden])
Date: 2004-08-04 15:45:26

Is there any reason for distinction between shared lock and upgradable 
shared lock? I mean - obviously updgradable shared lock can be upgraded 
to exclusive one, but do we need separate type for this? What's 
rationale behind this distinciton? I can easily imagine design (and 
implementation) where every shared lock is also upgradable one, if it's 
mutex type support upgrade operation. Under such design one can upgrade 
shared lock only if it's the only (currently locking) lock on 
synchromization object (supporting such transition). Implementation of 
such mutex could be very similar to semaphore (or just using semaphore, 
if present on platform).

Here are some simple use cases:

some_mutex_type_not_supporting_shared_locks  m1;
shared_lock<some...> l1(m1); // compilation error

some_mutex_type_fully_supporting_shared_locks  m2;
shared_lock<some...> l2(m2); // ok
exclusive_lock<some...> l3 = l2.try_upgrade(); // ok, non-blocking

some_mutex_type_with_limited_support_for_shared_locks  m3;
shared_lock<some...> l4(m3); // ok
exclusive_lock<some...> l5 = l4.try_upgrade(); // compilation error
exclusive_lock<some...> l6 = l4.upgrade(); // ok, blocking

some_mutex_type_with_minimal_support_for_shared_locks  m4;
{
   shared_lock<some...> l7(m4); // ok
   exclusive_lock<some...> l8 = l7.try_upgrade(); // compilation error
   exclusive_lock<some...> l9 = l7.upgrade(); // compilation error
}
exclusive_lock<some...> l10(m4); // ok

B.