On Thu, Feb 9, 2017 at 6:01 AM, Thomas Munro
<thomas.mu...@enterprisedb.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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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