[Boost-bugs] [Boost C++ Libraries] #8354: Only thread blocked in io_service.run() is not processing event loop.

Subject: [Boost-bugs] [Boost C++ Libraries] #8354: Only thread blocked in io_service.run() is not processing event loop.
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-03-30 17:52:16

#8354: Only thread blocked in io_service.run() is not processing event loop.
----------------------------------------------------+-----------------------
 Reporter:  Tanner Sansbury <twsansbury@…>          |       Owner:  chris_kohlhoff
     Type:  Bugs                                    |      Status:  new           
Milestone:  To Be Determined                        |   Component:  asio          
  Version:  Boost Development Trunk                 |    Severity:  Problem       
 Keywords:                                          |  
----------------------------------------------------+-----------------------
 There is a concurrency issue in {{{task_io_service}}} (Boost 1.48+), that
 can leave a thread blocked in {{{io_service.run()}}}, waiting for its
 {{{wakeup_event}}} to be signaled.  If thread A invokes
 {{{io_service::poll()}}}, runs the reactor while thread B invokes
 {{{io_service.run()}}}, becoming an idle thread ({{{op_queue_}}} is
 empty), then if no work is ready to run, thread A {{{io_service::poll()}}}
 returns {{{0}}} without waking up the idle thread B.  This results in
 thread B remaining blocked on {{{io_service::run()}}}, but never
 processing the event loop.

 A side-by-side visualization illustrating the problem:
 {{{
           poll thread                  |          main thread
 ---------------------------------------+---------------------------------------
   lock()                               |
   do_poll_one()                        |
   |-- pop task_operation_ from         |
   |   queue_op_                        |
   |-- unlock()                         | lock()
   |-- create task_cleanup              | do_run_one()
   |-- service reactor (non-block)      | `-- queue_op_ is empty
   |-- ~task_cleanup()                  |     |-- set thread as idle
   |   |-- lock()                       |     `-- unlock()
   |   `-- queue_op_.push(              |
   |       task_operation_)             |
   `-- task_operation_ is               |
       queue_op_.front()                |
       `-- return 0                     | // still waiting on wakeup_event
   unlock()                             |
 }}}

 Attached files are:
 * Patch file that modifies {{{task_io_service.ipp}}} to sleep immediately
 after {{{do_poll_one}}} runs the reactor.  This is only used to increase
 the changes of reproducing the concurrency problem.
 * Test application:
   * An asynchronous work loop via a timer that will print "." every 3
 seconds.
   * Spawn off a single thread that will poll the {{{io_service}}}.
   * Delay to allow the new thread time to poll {{{io_service}}}, and have
 main call {{{io_service::run()}} while the poll thread sleeps in
 {{{task_io_service::do_poll_one()}}}.
 * Patch with suggested fix that invokes {{{wake_one_thread_and_unlock()}}}
 before {{{do_poll_one}}} returning {{{0}}} if no work is to be done after
 running the reactor.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/8354>
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