Re: [Boost-bugs] [Boost C++ Libraries] #8730: Race condition in once_block.hpp

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #8730: Race condition in once_block.hpp
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-06-25 21:24:00

#8730: Race condition in once_block.hpp
-------------------------------+------------------------------------------
  Reporter:  apolukhin         |      Owner:  andysem
      Type:  Bugs              |     Status:  new
 Milestone:  To Be Determined  |  Component:  log
   Version:  Boost 1.54.0      |   Severity:  Problem
Resolution:                    |   Keywords:  call_once atomics thread log
-------------------------------+------------------------------------------

Comment (by andysem):

 Replying to [comment:4 apolukhin]:
 > {{{
 >     static boost::log::once_block_flag flag_var = {
 boost::log::once_block_flag::uninitialized };
 > }}}
 >
 > where `boost::log::once_block_flag` is a POD type.
 > Note the `static` keyword usage. It means that it will be converted by
 compiler to something like this:
 > {{{
 > if (!is_once_block_flag_inited) { // Variable constructed with compiler
 >     new (flag_var)
 boost::log::once_block_flag(boost::log::once_block_flag::uninitialized);
 >     is_once_block_flag_inited = true;
 > }
 > }}}

 No, it won't. Since it's POD, its initialization is performed during the
 static initialization stage, which happens before any dynamic
 initialization. See 3.6.2 [basic.start.init] in the standard.

 > That's only top of the iceberg. There are some more problems in
 `once_block_sentry::executed()`.

 What problems in particular?

 > What for is all the code inside once_block.cpp: you are using
 `boost::thread` but do not use `boost::call_once`? What is the reason?

 Once blocks offer more convenient syntax and are more lightweight than
 boost::call_once. BTW, boost::call_once also uses static initialization of
 its once_flag to achieve thread safety.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/8730#comment:5>
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