Re: Latch reset ordering bug in condition_variable.c

On Thu, Feb 9, 2017 at 6:01 AM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> ConditionVariablePrepareToSleep() has a race that can leave you
> hanging, introduced by me in the v4 patch. The problem is that that
> function resets our latch after adding our process to the wakeup list.
> With the right timing, the following sequence can happen:
>
> 1. ConditionVariablePrepareToSleep() adds us to the wakeup list.
> 2. Some other process calls ConditionVariableSignal(). It removes us
> from the wakeup list and sets our latch.
> 3. ConditionVariablePrepareToSleep() resets our latch.
> 4. We enter (or continue) our predicate loop. Our exit condition
> happens not to be true yet, so we call ConditionVariableSleep().
> 5. ConditionVariableSleep() never returns because WaitEventSet()
> blocks. Our latch is not set, yet we are no longer in the wakeup list
> so ConditionalVariableSignal() will never set it.
>
> We should reset the latch first. Then there is no way to reach
> ConditionVariableSleep() with neither a set latch nor an entry in the
> wakeup queue.
>
> See attached. Thoughts?

Oops. Committed.

--
Robert Haas
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