Date: Fri, 17 Oct 2008 19:44:58 -0400 (EDT) From: Daniel Eischen <deischen@freebsd.org> To: Kurt Miller <kurt@intricatesoftware.com> Cc: freebsd-gnats-submit@freebsd.org, freebsd-threads@freebsd.org Subject: Re: threads/128180: pthread_cond_broadcast() lost wakup Message-ID: <Pine.GSO.4.64.0810171941410.20136@sea.ntplx.net> In-Reply-To: <200810171640.m9HGexJ1090893@www.freebsd.org> References: <200810171640.m9HGexJ1090893@www.freebsd.org>
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On Fri, 17 Oct 2008, Kurt Miller wrote: > The test program outputs periodic printf's indicating > progress is being made. When it stops the process is > deadlocked. The lost wakeup can be confirmed by inspecting > the saved_waiters local var in main(). Each time the > deadlock occurs I see that saved_waiters is 8 which tells > me all eight worker threads were waiting on the condition > variable when the broadcast was sent. Then switch to the > thread that is still waiting on the condition variable, > and you can see that the last_cycle local var is one behind > the cycles global var which indicates it didn't receive the > last wakeup. The test program doesn't look correct to me. It seems possible for only a few of the threads (as little as 2) to do all the work. Thread 1 can start doing work, then wait for a broadcast. Thread 2 can start doing his work, then broadcast waking thread 1. I think you need separate condition variables, one to wake up the main thread when the last worker goes to sleep/finishes, and one to wake up the workers. -- DE
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