From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
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To: | Marko Kreen <markokr(at)gmail(dot)com> |
Cc: | Postgres Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: 8.3.5: Crash in CountActiveBackends() - lockless race? |
Date: | 2009-03-30 14:02:02 |
Message-ID: | 49D0D0DA.2000501@enterprisedb.com |
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Lists: | pgsql-hackers |
Marko Kreen wrote:
> On 3/30/09, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>> Agreed. And more importantly, it puts the onus of getting it right into
>> CountActiveBackends, which is the one who's breaking the rules. We don't
>> necessarily need to clear the pointer in ProcArrayRemove either, the count
>> doesn't need to be accurate.
>
> Without reset in ProcArrayRemove we may use some ancient pointer that
> may point to garbage? I don't think it's good coding style to allow
> that to happen.
Well, that can happen anyway. CountActiveBackends() could fetch the
pointer and determine that it's not NULL, just before ProcArrayRemove
clears it.
I agree it's a bit dirty, but seems safe as the PGPROC entries are in
shared memory.
(clearing the pointer might be a good idea anyway, though, for debugging
purposes)
> Also, are there other functions that try lockless access on proc_array?
We do set fields in MyProc without holding the lock, but that should be
fine.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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