| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru> |
| Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, YUriy Zhuravlev <u(dot)zhuravlev(at)postgrespro(dot)ru>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Move PinBuffer and UnpinBuffer to atomics |
| Date: | 2016-04-07 13:41:58 |
| Message-ID: | 20160407134158.a55ee4yyzfsht45j@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
On 2016-03-31 20:21:02 +0300, Alexander Korotkov wrote:
> ! BEGIN_BUFSTATE_CAS_LOOP(bufHdr);
>
> ! Assert(BUF_STATE_GET_REFCOUNT(state) > 0);
> ! wasDirty = (state & BM_DIRTY) ? true : false;
> ! state |= BM_DIRTY | BM_JUST_DIRTIED;
> ! if (state == oldstate)
> ! break;
I'm doubtful that this early exit is entirely safe. None of the
preceding operations imply a memory barrier. The buffer could previously
have been marked dirty, but cleaned since. It's pretty critical that we
re-set the dirty bit (there's no danger of loosing it with a barrier,
because we hold an exclusive content lock).
Practically the risk seems fairly low, because acquiring the exclusive
content lock will have implied a barrier. But it seems unlikely to have
a measurable performance effect to me, so I'd rather not add the early
exit.
Andres
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