Re: Spinlocks and compiler/memory barriers

On Thu, Jun 26, 2014 at 6:40 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> 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.

The relevant text is in barrier.h

-- 
Robert Haas
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