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POMultithreaded paranoia
 

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    <p>This is a complex question, please consider carefully before answering.</p> <p>Consider this situation. Two threads (a reader and a writer) access a single global <code>int</code>. Is this safe? Normally, I would respond without thought, yes!</p> <p>However, it seems to me that Herb Sutter doesn't think so. In his articles on effective concurrency he discusses a <a href="http://www.ddj.com/cpp/210600279" rel="nofollow noreferrer">flawed lock-free queue</a> and the <a href="http://www.ddj.com/hpc-high-performance-computing/210604448" rel="nofollow noreferrer">corrected version</a>.</p> <p>In the end of the first article and the beginning of the second he discusses a rarely considered trait of variables, write ordering. Int's are atomic, good, but ints aren't necessarily ordered which could destroy any lock-free algorithm, including my above scenario. I fully agree that the only way to <strong><em>guarantee</em></strong> correct multithreaded behavior on all platforms present and future is to use atomics(AKA memory barriers) or mutexes.</p> <p>My question; is write re-odering ever a problem on real hardware? Or is the multithreaded paranoia just being pedantic?<br> What about classic uniprocessor systems?<br> What about simpler RISC processors like an embedded power-pc?</p> <p><em>Clarification</em>: I'm more interested in what Mr. Sutter said about the hardware (processor/cache) reordering variable writes. I can stop the optimizer from breaking code with compiler switches or hand inspection of the assembly post-compilation. However, I'd like to know if the hardware can still mess up the code in practice.</p>
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    1. COInteresting. My first reaction is "if I cant be sure I wouldnt dare it"
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    2. COYep, same here. "Normally I would respond without thought, no!". It all depends what hardware you're used to - you can get away with just "volatile" on some platforms but not others. If you're used to writing portable code then you always assume the 'worst' possible behaviour.
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    3. CO... which in this case is multi-CPU with non-coherent memory caches. Then you must assume that changes made in one thread, even with volatile, will *never* propagate to other threads without a memory barrier. Rule of thumb: different threads run on different planets, and interact only when forced.
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