Re: Spinlocks and compiler/memory barriers

On 2014-06-27 13:04:02 -0400, Robert Haas wrote:
> On Thu, Jun 26, 2014 at 6:40 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> >> On 2014-06-26 14:13:07 -0700, Tom Lane wrote:
> >>> Surely it had better be a read barrier as well?
> >
> >> I don't immediately see why it has to be read barrier? Hoisting a load
> >> from after the release into the locked area of code should be safe?
> >
> > No doubt, but delaying a read till after the unlocking write would
> > certainly not be safe.
> >
> > AFAICT, README.barrier completely fails to define what we think the
> > semantics of pg_read_barrier and pg_write_barrier actually are, so if
> > you believe that a write barrier prevents reordering of reads relative to
> > writes, you'd better propose some new text for that file. It certainly
> > doesn't say that today.
>
> The relevant text is in barrier.h

Note that that definition of a write barrier is *not* sufficient for the
release of a lock... As I said elsewhere I think all the barrier
definitions, except maybe alpha, luckily seem to be strong enough for
that anyway.

Do we want to introduce acquire/release barriers? Or do we want to
redefine the current barriers to be strong enough for that?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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