Horton

Re: Condition variable live lock

Thomas Munro <thomas.munro@enterprisedb.com>

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From: Thomas Munro <thomas.munro@enterprisedb.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2018-01-08T11:41:36Z
Lists: pgsql-hackers 
On Mon, Jan 8, 2018 at 5:25 PM, Thomas Munro
<thomas.munro@enterprisedb.com> wrote:
> On Sun, Jan 7, 2018 at 10:43 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Actually ... perhaps a better design would be to have
>> ConditionVariable[PrepareTo]Sleep auto-cancel any prepared sleep for
>> a different condition variable, analogously to what we just did in
>> ConditionVariableBroadcast, on the same theory that whenever control
>> returns to the other CV wait loop it can re-establish the relevant
>> state easily enough.  I have to think that if the use of CVs grows
>> much, the existing restriction is going to become untenable anyway,
>> so why not just get rid of it?
>
> +1
>
> It's a more robust API this way.

One very small thing after another look:

-       Assert(cv_sleep_target == NULL);
+       if (cv_sleep_target != NULL)
+               ConditionVariableCancelSleep();

The test for cv_sleep_target != NULL is redundant since
ConditionVariableCancelSleep() would return early.
ConditionVariableBroadcast() doesn't do that.

-- 
Thomas Munro
http://www.enterprisedb.com

Commits

  1. Add WL_EXIT_ON_PM_DEATH pseudo-event.

  2. While waiting for a condition variable, detect postmaster death.

  3. Fix race condition during replication origin drop.

  4. Allow ConditionVariable[PrepareTo]Sleep to auto-switch between CVs.

  5. Cosmetic improvements in condition_variable.[hc].

  6. Improve error detection capability in proclists.

  7. Remove return values of ConditionVariableSignal/Broadcast.

  8. Reorder steps in ConditionVariablePrepareToSleep for more safety.

  9. Rewrite ConditionVariableBroadcast() to avoid live-lock.

  10. Add parallel-aware hash joins.