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    <p>You need a <code>volatile</code> qualifier on <code>lock</code>, and I would also make it a <code>sig_atomic_t</code>. Without the <code>volatile</code> qualifier, this code:</p> <pre><code> u16 savedlock = lock; while (savedlock == 0xffff || __sync_bool_compare_and_swap(&amp;lock, savedlock, savedlock+1) == false) savedlock = lock; </code></pre> <p>may not re-read <code>lock</code> when updating <code>savedlock</code> in the body of the while-loop. Consider the case that <code>lock</code> is 0xffff. Then, <code>savedlock</code> will be 0xffff prior to checking the loop condition, so the <code>while</code> condition will short-circuit prior to calling <code>__sync_bool_compare_and_swap</code>. Since <code>__sync_bool_compare_and_swap</code> wasn't called, the compiler doesn't encounter a memory barrier, so it might reasonably assume that the value of <code>lock</code> hasn't changed underneath you, and avoid re-loading it in <code>savedlock</code>.</p> <p>Re: <code>sig_atomic_t</code>, there's a decent discussion <a href="http://bytes.com/topic/c/answers/215161-sig_atomic_t" rel="nofollow noreferrer">here</a>. The same considerations that apply to signal handlers would also apply to threads.</p> <p>With these changes, I'd guess that your code would be thread-safe. I would still recommend using mutexes, though, since you really don't know how long your RB-tree insert will take in the general case (per my previous comments under the question).</p>
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    1. POLightweight spinlocks built from GCC atomic operations?
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    1. COThis is interesting. I have read many articles explaining why volatile is a multi-threaded program's best friend, and many explaining why volatile has nothing to do with this and making everything volatile will simply slow the program down. In my application, more than half of the data could be accessed by any thread and any time. Should they really all be volatile? Or is this the exception because it's in a tight loop that the compiler might optimize to only check the lock once?
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    2. COi.e. Picture a function (which isn't inlined) is called, checks a variable, then returns, and is called quickly again. In this case, would volatile not be necessary because the compiler would not be able to optimize code across multiple calls? But in the loop above it might realize that lock could never change and optimize it out? So volatile has nothing to do with caching, it simply tells the compiler not to optimize access to the memory? I think this just made sense to me. Please confirm or clarify! :)
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    3. COI spent some time looking up how volatile works... In a nutshell, what it does is to prevent optimizing memory accesses, and also prevent re-ordering of memory operations involving volatile variables. (Memory operations involving non-volatile-qualified variables may be re-ordered around those involving volatile. Further, even if the writes occur in order, a different CPU may notice the new values in a different order.) This should be enough for multi-threaded synchronization _in this case_, because you also have the `__sync` routines providing a memory barrier.
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