C:/rwlock/SG_RWLock.h

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// Stefan Gustafsson (sg@nospam.se) RW Lock
// http://groups.google.com/groups?hl=en&lr=&selm=%23l3RF6kO%24GA.267%40cppssbbsa04&rnum=24

#ifndef __SG_RWLock_H_Includded__
#define __SG_RWLock_H_Includded__

#include "atomic_util.h"

class SG_RWLock 
{
public:
        static const char *name() { return "SG_Lock"; }
        SG_RWLock()
        {
                readers = writers = 0;
        }
        void enter_read()
        {
                atomic_inc(&readers);

        // Wait until all writers have left
                while(writers > 0) thread_yield();
        }
        void leave_read()
        {
                atomic_dec(&readers);
        }
        void enter_write()
        {
                // Use a critsec for mutual exclusion between writers
                l.lock();
                // We must now wait until all readers have left the lock
                atomic_inc(&writers);
// -BUG         writers++; // Assume there are no readers
                while (readers > 0) 
                {
                        // Readers in the lock
                        // Decrement writers again to avoid deadlocking with readers that
                        // are interrupted immediately after incrementing readers
                        atomic_dec(&writers);
// - BUG                        writers--;
                        thread_yield();
                        atomic_inc(&writers);
// --BUG                        writers++; // Try again
                }
        }
        void leave_write()
        {   
                atomic_dec(&writers);
                l.unlock();
        }
private:
        volatile int readers; // The number of readers in the lock
        volatile int writers; // The number of writers in the lock
        SystemLock      l;
};

#endif // __SG_RWLock_H_Includded__

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