Is it valid to nest a critical section?

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( &a开发者_StackOverflow社区mp;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?

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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.

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From the documentation:

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.

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To validate the other two posts. Quick look at Critical section in WinDbg shows that cricital section maintains an integer variable to hold Recursion counts.

0:001> dt RTL_CRITICAL_SECTION
+0x000 DebugInfo : Ptr32 _RTL_CRITICAL_SECTION_DEBUG
+0x004 LockCount : Int4B
+0x008 RecursionCount : Int4B
+0x00c OwningThread : Ptr32 Void
+0x010 LockSemaphore : Ptr32 Void
+0x014 SpinCount : Uint4B 

RecursionCount - It is possible for a thread to acquire a critical section more than once. This field indicates how many times the same thread has acquired the critical section. By default, the value of this field is 0, indicating that there is no thread owning the critical section.