Re-Entrant Custom Critical Section

Bharath NS

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14 Nov 2010CPOL2 min read

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Re-Entrant code

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Introduction

The motivation to write this article came from Dr.Sai's excellent article "Working with CRITICAL_SECTION", so before reading this article one should first review that article.

This article explains how we can extend the custom critical section architecture to make it re-entrant.

Detail

Enter Part

void MyCriticalSection::Enter()

To make it re-entrant we need to have a variable to save the recursion count, m_lRecursionCount. The recursion count is local to the thread, is incremented each time that EnterCriticalSection is called and keeps count of the number of times that EnterCriticalSection is called in the thread.

In the above scenario, the expression if (m_ThreadID == GetCurrentThreadId()) will evalute to TRUE and increment the recursion count. Prior to that, m_threadID is made equal to GetCurrentThreadId() once either interlockedincrement(&count) == 0 or waitforsingleobject succeeds.

Leave Part

void MyCriticalSection::Leave()

The expression if (m_ThreadID != GetCurrentThreadId()) is necessary so that no other thread makes a call to Leave() without making a call to Enter(), in which case the call to Leave should be ignored.

First a check of m_lrecursivecount == 0 is made. If the recursion count is not zero, the recursion count is decremented, which means the thread needs to call Leave some additional times before relinquishing control to other waiting threads.

If the statement m_lrecursivecount == 0 evaluates to TRUE, then we need to decrement the count and make a call to SetEvent. Before making a call to SetEvent we need to check that InterlockedDecrement(&count) > 0. If TRUE, a call to SetEvent is made; if FALSE, it is the last thread and hence there is no need to call SetEvent.