On Thu, Jun 26, 2014 at 12:20:06PM +0200, Andres Freund 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.
I agree it's valuable to have a design that permits invoking an atomic operation while holding a spinlock. > > > I added the x86 inline assembler because a fair number of buildfarm > > > animals use ancient gcc's and because I could easily test it. It's also > > > more efficient for gcc < 4.6. I'm not wedded to keeping it. > > > > Hmm. gcc 4.6 is only just over a year old, so if pre-4.6 > > implementations aren't that good, that's a pretty good argument for > > keeping our own implementations around. :-( GCC 4.6 was released 2011-03-25, though that doesn't change the bottom line. -- Noah Misch EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
