On 2014-06-26 07:44:14 -0500, Merlin Moncure wrote: > On Thu, Jun 26, 2014 at 5:20 AM, Andres Freund <and...@2ndquadrant.com> wrote: > > On 2014-06-25 20:16:08 -0400, Robert Haas wrote: > >> On Wed, Jun 25, 2014 at 4:36 PM, Andres Freund <and...@2ndquadrant.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 :). > > > > More seriously, I think we're not going to have much fun if we're making > > up the rule that you can't do an atomic add/sub while a spinlock is > > held. That just precludes to many use cases and will make the code much > > harder to understand. I don't think we're going to end up having many > > problems if we allow atomic read/add/sub/write in there. > > That rule seems reasonable -- why would you ever want to do this?
Are you wondering why you'd ever manipulate an atomic op inside a spinlock? There's enough algorithms where a slowpath is done under a spinlock but the fastpath is done without. You can't simply use nonatomic operations for manipulation under the spinlock because the fastpaths might then observe/cause bogus state. Obviously I'm not advocating to do random stuff in spinlocks. W're talking about single lock xadd;s (or the LL/SC) equivalent here. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers