Re: better atomics - v0.5 - Mailing list pgsql-hackers

On 2014-06-26 11:47:15 -0700, Tom Lane wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > On Thu, Jun 26, 2014 at 6:20 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> >> On 2014-06-25 20:16:08 -0400, Robert Haas wrote:
> >>> 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.
> 
> Well, the point here is to be sure that there's a reasonably small bound
> on how long we hold the spinlock.  It doesn't have to be zero, just small.
> 
> Would it be practical to say that the coding rule is that you can only
> use atomic ops on fields that are protected by the spinlock, ie, nobody
> else is supposed to be touching those fields while you have the spinlock?
> If that's the case, then the atomic op should see no contention and thus
> not take very long.

I don't really see usecases where it's not related data that's being
touched, but: The point is that the fastpath (not using a spinlock) might
touch the atomic variable, even while the slowpath has acquired the
spinlock. So the slowpath can't just use non-atomic operations on the
atomic variable.
Imagine something like releasing a buffer pin while somebody else is
doing something that requires holding the buffer header spinlock.

> On the other hand, if the atomic op is touching something not protected
> by the spinlock, that seems to me to be morally equivalent to taking a
> spinlock while holding another one, which as Robert says is forbidden
> by our current coding rules, and for very good reasons IMO.
>
> However, that may be safe only as long as we have real atomic ops.
> If we're simulating those using spinlocks then you have to ask questions
> about how many underlying spinlocks there are and whether artificial
> contention might ensue (due to the same spinlock being used for multiple
> things).

With the code as submitted it's one spinlock per atomic variable. Which
is fine until spinlocks are emulated using semaphores - in that case a
separate array of semaphores (quite small in the patch as submitted) is
used so there's no chance of deadlocks against a 'real' spinlock or
anything like that.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services