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Critical sections support recursive acquires That is to say, if the current thread holds the section when E nt e r C r it i c a l Se c t i o n is called, an internal recursion counter is incremented and the acquisition immediately succeeds When LeaveC r i t i c a lSection is subsequently called, the recursion counter is decremented by 1; only when this counter reaches 0 is the section actually exited, made available to other threads, and any waiting threads awakened Recursion is possible because the critical section tracks ownership informa tion, enabling it to determine whether the calling thread is the current owner While recursion may seem like a generally convenient feature, it does come with some unique challenges because it is very easy to accidentally recur sively acquire a lock and depend (incorrectly) on certain state invariants holding We review this issue more in 1 1 , Concurrency Hazards
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Leaving an Unowned Critica l Section It is a very serious bug to try to leave a critical section that isn't owned by the current thread In all cases, this indicates a programming error, and, if it ever occurs, there is no imme diate indication that something has gone wrong There is no error code or exception Despite the appearance that all is well, a ticking time bomb has been left behind If the critical section is completely unowned at the time of the erroneous call to L e a v e C r i t i c a l S e c t i o n , all future calls to E nt e r C r it i c a l S e c t i o n will block forever This effectively deadlocks all threads that later try t o use this critical section If the section is owned by another thread when the unowning thread tries to leave it, the current owner is still permitted to reacquire and release the lock recursively But once the owner exits the lock completely, the lock has become permanently damaged: subsequent behav ior is identical to the case where no owner was initially present In other words, all subsequent calls to E nt e r C r it i c a l S e c t i o n by any thread in the system will block indefinitely Ensuring a Thread Always Leaves the Critical Section We usually want to ensure LeaveC r it i c a l S e c t i o n is called no matter the outcome of the crit ical region itself Please first recall the warnings about reliability and the possibility of leaving corrupt state in the wake of an unhandled exception
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stemming from a critical region Assuming we're convinced w e d o want this behavior, we can use a try / finally block
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E n t e r C r it i c a lS e c t ion ( &m_c r st ) ; _t ry
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LeaveC r i t i c a lS e c t ion ( &m_c r st ) ;
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While this certainly does the trick and is a fairly simple pattern to follow, it' s easy to accidentally slip in a call to some function that might throw exceptions after the E nt e r C r it i c a l S e c t i o n but before the try block If an exception were thrown from such a function, the finally block will not run, leading to an orphaned lock and subsequent deadlocks Instead of writing this boilerplate everywhere, we can use a C++ holder type (see Further Reading, Meyers) A holder is a stack allocated object that manages a resource and takes advantage of C++'s implicit destructor invocation at the end of the scope in which it' s used for cleanup
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# i n c l u d e <windows h > c l a s s C r stHolder LPCRITICAL_S ECTION m_pC rst ; public : C r stHolder ( LPCR ITICAL_S E CTION pCrst )
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E nterCrit i c a lSection ( m_pC rst ) ; } -CrstHolde r ( )
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LeaveC r i t i c a lSection ( m_pC r st ) ;
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Allocating a holder and deleting it will perform lock acquisition and release, respectively This holder can then be used anywhere we need to create a critical region For example, we can now go ahead and change our try / finally example to use the holder instead
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