Dear C++ memory model experts,
yesterday I skimmed through the sources of the latest Boost.Thread
library, especially the POSIX Threads implementation of
boost::call_once(). It fundamentally changed in the past. The current
implementation quotes the ISO/IEC JTC1 SC22 WG21 N2444 document
(<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2444.html>)
and uses an algorithm by Mike Burrow published there.
After reading especially Meyers / Alexandrescu: "C++ and the Perils of
Double-Checked Locking" (<http://www.aristeia.com/Papers/
DDJ_Jul_Aug_2004_revised.pdf>) (and for Java: Bacon et al.: "The
'Double-Checked Locking is Broken' Declaration" (<http://
www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html>)) I
want to ask the following:
1. Following several threads on the atomicity with regard to signals
and non-atomicity with regard to threads of sig_atomic_t (or another
integral type in the Boost implementation) my question is as follows:
Can the [atomicity] and [atomicity2] requirements really be portably
fulfilled on POSIX Threads systems? But do we really need this
requirement at all for the proof?
2. I miss one argument in the correctness reasoning layed out by
Burrow: "_fast_pthread_once_per_thread_epoch" (line 2) implies a
memory barrier. He mentions that the second
"pthread_mutex_lock()" (line 8) is assumed to provide release
consistency. So in total we end up with the two barriers declared
necessary on Page 12 (the multiprocessor Section) of the Meyers /
Alexandrescu paper. Or do I miss something here?
So, I believe the code is correct, but as the correctness of such a
central piece of software is vital, someone - including me - should
take the time to contribute to such a "mental exercise" (Burrow).
Cheers,
Philipp.
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