Re: better atomics - v0.5

On Thu, Jun 26, 2014 at 6:20 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> On 2014-06-25 20:16:08 -0400, Robert Haas wrote:
>> On Wed, Jun 25, 2014 at 4:36 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>> > Since it better be legal to manipulate a atomic variable while holding a
>> > spinlock we cannot simply use an arbitrary spinlock as backing for
>> > atomics. That'd possibly cause us to wait on ourselves or cause
>> > deadlocks.
>>
>> I think that's going to fall afoul of Tom's previously-articulated "no
>> loops inside spinlocks" rule. Most atomics, by nature, are
>> loop-until-it-works.
>
> Well, so is TAS itself :).

Yeah, which is why we have a rule that you're not suppose to acquire
another spinlock while already holding one. You're trying to make the
spinlocks used to simulate atomic ops an exception to that rule, but
I'm not sure that's OK. An atomic op is a lot more expensive than a
regular unlocked load or store even if it doesn't loop, and if it does
loop, it's worse, potentially by a large multiple.

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