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

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.

> ... 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.

Now, I did think of a problem we'd have to deal with, which is how
to avoid breaking ABI by changing sizeof(PGSemaphoreData). I think
that's soluble though.

regards, tom lane