On Fri, Apr 8, 2016 at 7:39 PM, Andres Freund <andres@anarazel.de> wrote:
On 2016-04-07 16:50:44 +0300, Alexander Korotkov wrote: > On Thu, Apr 7, 2016 at 4:41 PM, Andres Freund <andres@anarazel.de> wrote: > > > 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). > > > > Oh, I get it. > > > > 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. > > > > Ok, let's just remove it.
Here's my updated version of the patch. I've updated this on an intercontinental flight, after a otherwise hectic week (moving from SF to Berlin); so I'm planning to look over this once more before pushing (.
Ok.
I've decided that the cas-loop macros are too obfuscating for my taste. To avoid duplicating the wait part I've introduced WaitBufHdrUnlocked().
That's OK for me. Cas-loop macros looks cute, but too magic.