| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
| Cc: | pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Spinlocks and compiler/memory barriers |
| Date: | 2014-06-26 22:40:11 |
| Message-ID: | 78643.1403822411@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
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.
regards, tom lane
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