cache invalidation skip logic
In cache invalidation logic, we have the following comment:
/*
* Now that we have the lock, check for invalidation messages, so that we
* will update or flush any stale relcache entry before we try to use it.
* RangeVarGetRelid() specifically relies on us for this. We can skip
* this in the not-uncommon case that we already had the same type of lock
* being requested, since then no one else could have modified the
* relcache entry in an undesirable way. (In the case where our own xact
* modifies the rel, the relcache update happens via
* CommandCounterIncrement, not here.)
*/
if (res != LOCKACQUIRE_ALREADY_HELD)
AcceptInvalidationMessages();
It is true after we hold the lock, nobody will further modify it but there
could be some left-over invalidation message we shall accept before we can
continue. This is can be demonstrated with the following invalidation
sequence:
{
1: inval A;
2: inval B;
...;
10: inval pg_class
}
After step 10, another session may encounter a lock and replays this sequence:
step 1: RelationBuildDesc(A), it heap_open(pg_class),
pg_class lock not acquired yet, so it acquires the lock and
recursively replay the sequence, goto step 2.
step 2:
RelationBuildDesc(B), it heap_open(pg_class),
but this time we already have LOCKACQUIRE_ALREADY_HELD with
pg_class, so we now access pg_class but it is wrong.
User may ends up with a "could not open file ..." error.
Is above sequence possible?
Regards,
Qingqing
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On Thu, Aug 6, 2015 at 2:19 PM, Qingqing Zhou <zhouqq.postgres@gmail.com> wrote:
In cache invalidation logic, we have the following comment:
/*
* Now that we have the lock, check for invalidation messages, so that we
* will update or flush any stale relcache entry before we try to use it.
* RangeVarGetRelid() specifically relies on us for this. We can skip
* this in the not-uncommon case that we already had the same type of lock
* being requested, since then no one else could have modified the
* relcache entry in an undesirable way. (In the case where our own xact
* modifies the rel, the relcache update happens via
* CommandCounterIncrement, not here.)
*/
if (res != LOCKACQUIRE_ALREADY_HELD)
AcceptInvalidationMessages();It is true after we hold the lock, nobody will further modify it but there
could be some left-over invalidation message we shall accept before we can
continue. This is can be demonstrated with the following invalidation
sequence:
{
1: inval A;
2: inval B;
...;
10: inval pg_class
}After step 10, another session may encounter a lock and replays this sequence:
step 1: RelationBuildDesc(A), it heap_open(pg_class),
pg_class lock not acquired yet, so it acquires the lock and
recursively replay the sequence, goto step 2.
step 2:
RelationBuildDesc(B), it heap_open(pg_class),
but this time we already have LOCKACQUIRE_ALREADY_HELD with
pg_class, so we now access pg_class but it is wrong.User may ends up with a "could not open file ..." error.
Is above sequence possible?
In step 1, AcceptInvalidationMessages() should process all pending
invalidation messages. So if step 2 did AcceptInvalidationMessages()
again it would be a no-op, because no messages should remain at that
point.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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On Sun, Aug 9, 2015 at 8:24 AM, Robert Haas <robertmhaas@gmail.com> wrote:
In step 1, AcceptInvalidationMessages() should process all pending
invalidation messages. So if step 2 did AcceptInvalidationMessages()
again it would be a no-op, because no messages should remain at that
point.
That's what I think at first. I would try to see if I can manually repro a case.
Thanks,
Qingqing
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