Re: Reviewing freeze map code

On Tue, Jun 21, 2016 at 3:46 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2016-06-21 15:38:25 -0400, Robert Haas wrote:
>> On Tue, Jun 21, 2016 at 1:49 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> >> I'm also a bit dubious that LockAcquire is safe to call in general
>> >> with interrupts held.
>> >
>> > Looks like we could just acquire the tuple-lock *before* doing the
>> > toast_insert_or_update/RelationGetBufferForTuple, but after releasing
>> > the buffer lock. That'd allow us to do avoid doing the nested locking,
>> > should make the recovery just a goto l2;, ...
>>
>> Why isn't that racey? Somebody else can grab the tuple lock after we
>> release the buffer content lock and before we acquire the tuple lock.
>
> Sure, but by the time the tuple lock is released, they'd have updated
> xmax. So once we acquired that we can just do
> if (xmax_infomask_changed(oldtup.t_data->t_infomask,
> infomask) ||
> !TransactionIdEquals(HeapTupleHeaderGetRawXmax(oldtup.t_data),
> xwait))
> goto l2;
> which is fine, because we've not yet done the toasting.

I see.

> I'm not sure wether this approach is better than deleting potentially
> toasted data though. It's probably faster, but will likely touch more
> places in the code, and eat up a infomask bit (infomask & HEAP_MOVED
> == HEAP_MOVED in my prototype).

Ugh. That's not very desirable at all.

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