On 3/30/09, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> 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
> > > 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.
Yes, but that way it's well-defined what pointer you can get there -
it can only be a pointer that is just being removed.
And you know that if the ProcArrayRemove does not invalidate any fields
before LWunlock, the struct data is valid.
> I agree it's a bit dirty, but seems safe as the PGPROC entries are in
> shared memory.
I understand that the pointer is valid, but what about data
it is pointing to?
> (clearing the pointer might be a good idea anyway, though, for debugging
Yes, I think it would be good idea.
> > 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
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