Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

On Fri, Dec 8, 2017 at 3:20 AM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>> The first hunk in monitoring.sgml looks unnecessary.
>
> You meant the following hunk?
>
> diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
> index 8d461c8..7aa7981 100644
> --- a/doc/src/sgml/monitoring.sgml
> +++ b/doc/src/sgml/monitoring.sgml
> @@ -669,8 +669,8 @@ postgres 27093 0.0 0.0 30096 2752 ?
> Ss 11:34 0:00 postgres: ser
> Heavyweight locks, also known as lock manager locks or simply locks,
> primarily protect SQL-visible objects such as tables. However,
> they are also used to ensure mutual exclusion for certain internal
> - operations such as relation extension.
> <literal>wait_event</literal> will
> - identify the type of lock awaited.
> + operations such as waiting for a transaction to finish.
> + <literal>wait_event</literal> will identify the type of lock awaited.
> </para>
> </listitem>
> <listitem>
>
> I think that since the extension locks are no longer a part of
> heavyweight locks we should change the explanation.

Yes, you are right.

>> +RelationExtensionLockWaiterCount(Relation relation)
>>
>> Hmm. This is sort of problematic, because with then new design we
>> have no guarantee that the return value is actually accurate. I don't
>> think that's a functional problem, but the optics aren't great.
>
> Yeah, with this patch we could overestimate it and then add extra
> blocks to the relation. Since the number of extra blocks is capped at
> 512 I think it would not become serious problem.

How about renaming it EstimateNumberOfExtensionLockWaiters?

>> + /* If force releasing, release all locks we're holding */
>> + if (force)
>> + held_relextlock.nLocks = 0;
>> + else
>> + held_relextlock.nLocks--;
>> +
>> + Assert(held_relextlock.nLocks >= 0);
>> +
>> + /* Return if we're still holding the lock even after computation */
>> + if (held_relextlock.nLocks > 0)
>> + return;
>>
>> I thought you were going to have the caller adjust nLocks?
>
> Yeah, I was supposed to change so but since we always release either
> one lock or all relext locks I thought it'd better to pass a bool
> rather than an int.

I don't see why you need to pass either one. The caller can set
held_relextlock.nLocks either with -- or = 0, and then call
RelExtLockRelease() only if the resulting value is 0.

>> When I took a break from sitting at the computer, I realized that I
>> think this has a more serious problem: won't it permanently leak
>> reference counts if someone hits ^C or an error occurs while the lock
>> is held? I think it will -- it probably needs to do cleanup at the
>> places where we do LWLockReleaseAll() that includes decrementing the
>> shared refcount if necessary, rather than doing cleanup at the places
>> we release heavyweight locks.
>> I might be wrong about the details here -- this is off the top of my head.
>
> Good catch. It can leak reference counts if someone hits ^C or an
> error occurs while waiting. Fixed in the latest patch. But since
> RelExtLockReleaseAll() is called even when such situations I think we
> don't need to change the place where releasing the all relext lock. We
> just moved it from heavyweight locks. Am I missing something?

Hmm, that might be an OK way to handle it. I don't see a problem off
the top of my head. It might be clearer to rename it to
RelExtLockCleanup() though, since it is not just releasing the lock
but also any wait count we hold.

+/* Must be greater than MAX_BACKENDS - which is 2^23-1, so we're fine. */
+#define RELEXT_WAIT_COUNT_MASK ((uint32) ((1 << 24) - 1))

Let's drop the comment here and instead add a StaticAssertStmt() that
checks this.

I am slightly puzzled, though. If I read this correctly, bits 0-23
are used for the waiter count, bit 24 is always 0, bit 25 indicates
the presence or absence of an exclusive lock, and bits 26+ are always
0. That seems slightly odd. Shouldn't we either use the highest
available bit for the locker (bit 31) or the lowest one (bit 24)? The
former seems better, in case MAX_BACKENDS changes later. We could
make RELEXT_WAIT_COUNT_MASK bigger too, just in case.

+ /* Make a lock tag */
+ tag.dbid = MyDatabaseId;
+ tag.relid = relid;

What about shared relations? I bet we need to use 0 in that case.
Otherwise, if backends in two different databases try to extend the
same shared relation at the same time, we'll (probably) fail to notice
that they conflict.

+ * To avoid unnecessary recomputations of the hash code, we try to do this
+ * just once per function, and then pass it around as needed. we can
+ * extract the index number of RelExtLockArray.

This is just a copy-and-paste from lock.c, but actually we have a more
sophisticated scheme here. I think you can just drop this comment
altogether, really.

+ return (tag_hash((const void *) locktag, sizeof(RelExtLockTag))
+ % N_RELEXTLOCK_ENTS);

I would drop the outermost set of parentheses. Is the cast to (const
void *) really doing anything?

+ "cannot acquire relation extension locks for
multiple relations at the same");

cannot simultaneously acquire more than one distinct relation lock?
As you have it, you'd have to add the word "time" at the end, but my
version is shorter.

+ /* Sleep until the lock is released */

Really, there's no guarantee that the lock will be released when we
wake up. I think just /* Sleep until something happens, then recheck
*/

+ lock_free = (oldstate & RELEXT_LOCK_BIT) == 0;
+ if (lock_free)
+ desired_state += RELEXT_LOCK_BIT;
+
+ if (pg_atomic_compare_exchange_u32(&relextlock->state,
+ &oldstate, desired_state))
+ {
+ if (lock_free)
+ return false;
+ else
+ return true;
+ }

Hmm. If the lock is not free, we attempt to compare-and-swap anyway,
but then return false? Why not just lock_free = (oldstate &
RELEXT_LOCK_BIT) == 0; if (!lock_free) return true; if
(pg_atomic_compare_exchange(&relextlock->state, &oldstate, oldstate |
RELEXT_LOCK_BIT)) return false;

+ Assert(IsAnyRelationExtensionLockHeld() == 0);

Since this is return bool now, it should just be
Assert(!IsAnyRelationExtensionLockHeld()).

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company