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copied!<p>One of the issues is the definition of "memory location", that allows (and forces the compiler to support) locking different structure members by different locks. There is a <a href="http://gcc.gnu.org/ml/gcc/2012-02/msg00005.html" rel="noreferrer">discussion about a RL problem caused by this</a>.</p> <p>Basically the issue is that having a <code>struct</code> defined like this:</p> <pre><code>struct x { long a; unsigned int b1; unsigned int b2:1; }; </code></pre> <p>the compiler is free to implement writing to <code>b2</code> by overwriting <code>b1</code> too (and apparently, judging from the report, it does). Therefore, the two fields have to be locked as one. However, as a consequence of the C++11 memory model, this is forbidden (well, not really forbidden, but the compiler must ensure simultaneous updates to <code>b1</code> and <code>b2</code> do not interfere; it could do it by locking or CAS-ing each such update, well, life is difficult on some architectures). Quoting from the report:</p> <blockquote> <p>I've raised the issue with our GCC guys and they said to me that: "C does not provide such guarantee, nor can you reliably lock different structure fields with different locks if they share naturally aligned word-size memory regions. The C++11 memory model would guarantee this, but that's not implemented nor do you build the kernel with a C++11 compiler."</p> </blockquote> <p>Nice info can also be found in the <a href="http://gcc.gnu.org/wiki/Atomic/GCCMM" rel="noreferrer">wiki</a>.</p>

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