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POShould access to a shared resource be locked by a parent thread before spawning a child thread that accesses it?
 

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  1. POShould access to a shared resource be locked by a parent thread before spawning a child thread that accesses it?
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    <p>If I have the following psuedocode:</p> <p><code>sharedVariable = somevalue;</code><br> <code>CreateThread(threadWhichUsesSharedVariable);</code></p> <p>Is it theoretically possible for a multicore CPU to execute code in threadWhichUsesSharedVariable() which reads the value of sharedVariable before the parent thread writes to it? For full theoretical avoidance of even the remote possibility of a race condition, should the code look like this instead:</p> <p><code>sharedVariableMutex.lock();</code><br> <code>sharedVariable = somevalue;</code><br> <code>sharedVariableMutex.unlock();</code><br> <code>CreateThread(threadWhichUsesSharedVariable);</code></p> <p>Basically I want to know if the spawning of a thread explicitly linearizes the CPU at that point, and is guaranteed to do so.</p> <p>I know that the overhead of thread creation probably takes enough time that this would never matter in practice, but the perfectionist in me is afraid of the theoretical race condition. In extreme conditions, where some threads or cores might be severely lagged and others are running fast and efficiently, I can imagine that it might be remotely possible for the order of execution (or memory access) to be reversed unless there was a lock.</p>
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    1. USDeadcode
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    1. COMy question concerns languages like C++, where you're close to the machine level, and where a "lock" is achieved using a simple InterlockedExchange loop. I'm concerned that declaring variables "volatile" is not enough to ensure synchronization. Isn't there a difference between "atomic" and "synchronized"? Atomic operations can't be split up — and if a variable is declared volatile, the compiler won't reorder accesses to it — but can't the CPU reorder them? Especially when accessed by different cores?
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    2. COAdditional clarification: In my example, "sharedVariable = somevalue" is used to pass data to threadWhichUsesSharedVariable. It is very important that threadWhichUsesSharedVariable only accesses "sharedVariable" after it has been assigned by the parent thread.
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    3. COThe CPU can only reorder the accesses if the reordering is not visible to the program so CPU reordering should not be a concern. Much of the complexity in designing multi-core system is in ensuring that the programs never see a difference in behaviour from a single-core system.
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      1. USPer Knytt
 

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