Re: Back-patch use of unnamed POSIX semaphores for Linux?

On Tue, Dec 6, 2016 at 11:54 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Tue, Dec 6, 2016 at 9:53 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> I think we should give serious consideration to back-patching commit
>>> ecb0d20a9, which changed the default semaphore type to unnamed-POSIX
>>> on Linux.
>
>> Urk. That sounds like a scary thing to back-patch.
>
> I don't deny that it's scary, but the alternative seems to be to be
> rather badly broken on systemd-using distros for years to come.
> That's pretty scary too.

Why can't this be configurable?

>> ... Granted, that might not
>> happen, because maybe unnamed POSIX semas are one of those really
>> awesome operating system primitives that never has problems on any
>> system anywhere ever. But I think it's pretty hard to be certain of
>> that.
>
> You're attacking a straw man. I didn't propose changing our behavior
> anywhere but Linux. AFAIK, on that platform unnamed POSIX semaphores
> are futexes, which have been a stable feature since 2003 according to
> https://en.wikipedia.org/wiki/Futex#History. Anybody who did need
> to compile PG for use with a pre-2.6 kernel could override the default,
> anyway.

Changing the behavior even just on Linux leaves plenty of room for
failure, even if the feature itself has been stable. For example,
there are Linux machines where POSIX shared memory doesn't work, even
though POSIX shared memory is in general a supported feature on Linux
and has been for a long time. So, if we were to change from System V
shared memory to POSIX shared memory in a minor release, anyone in
that situation would break. It's hard to be sure the same thing
wouldn't happen in this case. The fact that the feature's stable
doesn't prove that it works on every system in every configuration.

--
Robert Haas
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