Question

For example, would this be valid?

CRITICAL_SECTION cs;

::InitializeCriticalSection( &cs );

::EnterCriticalSection( &cs );      // First level
::EnterCriticalSection( &cs );        // Second level

/* do some stuff */

::LeaveCriticalSection( &cs );        // Second level
::LeaveCriticalSection( &cs );      // First level

::DeleteCriticalSection( &cs );

Obviously, I would never intentionally do this, but what if this were to come about as a result of function calls such that the “first level” gets called to lock an object for a complex (e.g. search) algorithm and the “second level” gets called in that object’s accessor functions?

Answer 1

Yes it is valid to enter the same critical section while already inside it. From the docs:

After a thread has ownership of a critical section, it can make
additional calls to EnterCriticalSection or TryEnterCriticalSection
without blocking its execution. This prevents a thread from
deadlocking itself while waiting for a critical section that it
already owns. The thread enters the critical section each time
EnterCriticalSection and TryEnterCriticalSection succeed. A thread
must call LeaveCriticalSection once for each time that it entered the
critical section.