Re: [BUG FIX] Removing NamedLWLockTrancheArray

On Fri, Mar 3, 2017 at 1:43 PM, Kyotaro HORIGUCHI
<horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> Hello, some of my collegues found that orafce crashes with
> postgresql compliled with dtrace.
>
> === The cause
>
> The immediate cause was I think that it just did
> LWLockNewTrancheId and forget to do LWLockRegisterTranche. But
> this is hardly detectable by a module developer.
>
>
> Typical tracepoint looks like the following.
>
>> TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
>
> Where T_NAME is (LWLockTrancheArray[(lock)->tranche]), as you see
> T_NAME assumes that all tranches pointed from lwlocks are
> available on the array but the tranches without
> LWLockRegisterTranche are not. What is worse this doesn't harm so
> much without dtrace (or LWLOCK_STATS or error in LWLockRelease)
> . Even if dtrace is activated one or two unregistred tranches
> (faultly) have their seat (filled with trash) at the end of the
> array with the current code.
>
> As my understanding there are two ways to use lwlocks in
> extension. One is using LWLockNewTrancheId and
> LWLockRegisterTanche on shared memory provided by the module. The
> other is using RequestNamedLWLockTranche in _PG_init and
> GetNamedLWLockTranche to acquire locks provided by postgresql.
>
> I couldn't find a documentation about lwlock and trance in
> extentions, is there any?
>

You can read about usage of LWLocks in extensions from below location:
https://www.postgresql.org/docs/devel/static/xfunc-c.html#idp86986416

Alternatively, you can check the commit
c1772ad9225641c921545b35c84ee478c326b95e to see how it is used in
pg_stat_statements.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com