Re: Spinlocks and compiler/memory barriers

Andres Freund <andres@2ndquadrant.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.
        regards, tom lane