Re: foreign key locks

On 2012-11-14 13:27:26 -0300, Alvaro Herrera wrote:
> Alvaro Herrera wrote:
>
> > > * In heap_lock_tuple's XMAX_IS_MULTI case
> > >
> > > [snip]
> > >
> > > why is it membermode > mode and not membermode >= mode?
> >
> > Uh, that's a bug. Fixed. As noticed in the comment above that snippet,
> > there was a deadlock possible here. Maybe I should add a test to ensure
> > this doesn't happen.
>
> Done:
> https://github.com/alvherre/postgres/commit/df2847e38198e99f57e52490e1e9391ebb70d770

Some more review bits, based on ffd6250d1d393f2ecb9bfc55c2c6f715dcece557

- if oldestMultiXactId + db is set and then that database is dropped we seem to
have a problem because MultiXactAdvanceOldest won't overwrite those
values. Should probably use SetMultiXactIdLimit directly.

- what stop multixacts only being filled out (i.e RecordNewMultiXact()) *after*
the XLogInsert() *and* after a MultiXactGetCheckptMulti()? Afaics
MultiXactGenLock is not hold in CreateMultiXactId(). If we crash in that
moment we loose the multixact data which now means potential data loss...

- multixact member group data crossing 512 sector boundaries makes me uneasy
(as its 5 bytes). I don't really see a scenario where its dangerous, but
... Does anybody see a problem here?

- there are quite some places that do
multiStopLimit = multiWrapLimit - 100;
if (multiStopLimit < FirstMultiXactId)
multiStopLimit -= FirstMultiXactId;

perhaps MultiXactIdAdvance and MultiXactIdRetreat macros are in order?

- I find using a default: clause in switches with enum types where everything
is expected to be handled like the following a bad idea, this way the
compiler won't warn you if youve missed case's which makes changing/extending code harder:
switch (rc->strength)
{
case LCS_FORNOKEYUPDATE:
newrc->markType = ROW_MARK_EXCLUSIVE;
break;
case LCS_FORSHARE:
newrc->markType = ROW_MARK_SHARE;
break;
case LCS_FORKEYSHARE:
newrc->markType = ROW_MARK_KEYSHARE;
break;
case LCS_FORUPDATE:
newrc->markType = ROW_MARK_KEYEXCLUSIVE;
break;
default:
elog(ERROR, "unsupported rowmark type %d", rc->strength);
}
-
#if 0
/*
* The idea here is to remove the IS_MULTI bit, and replace the
* xmax with the updater's Xid. However, we can't really do it:
* modifying the Xmax is not allowed under our buffer locking
* rules, unless we have an exclusive lock; but we don't know that
* we have it. So the multi needs to remain in place :-(
*/
ResetMultiHintBit(tuple, buffer, xmax, true);
#endif

Three things:
- HeapTupleSatisfiesUpdate is actually always called exclusively locked ;)
- Extending something like LWLockHeldByMe to also return the current
lockmode doesn't sound hard
- we seem to be using #ifdef NOT_YET for such cases

- Using a separate production for the lockmode seems to be nicer imo, not
really important though
for_locking_item:
FOR UPDATE locked_rels_list opt_nowait
...
| FOR NO KEY UPDATE locked_rels_list opt_nowait
...
| FOR SHARE locked_rels_list opt_nowait
...
| FOR KEY SHARE locked_rels_list opt_nowait
;

- not really padding, MultiXactStatus is 4bytes...
/*
* XXX Note: there's a lot of padding space in MultiXactMember. We could
* find a more compact representation of this Xlog record -- perhaps all the
* status flags in one XLogRecData, then all the xids in another one? Not
* clear that it's worth the trouble though.
*/
- why
#define SizeOfMultiXactCreate (offsetof(xl_multixact_create, nmembers) + sizeof(int32))
and not
#define SizeOfMultiXactCreate offsetof(xl_multixact_create, members)
- starting a critical section in GetNewMultiXactId but not ending it is ugly,
but not new

Greetings,

Andres

--
Andres Freund http://www.2ndQuadrant.com/
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