From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Back-patch use of unnamed POSIX semaphores for Linux?
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Tue, 06 Dec 2016 23:43:56 -0500"
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Date: Tue, 06 Dec 2016 23:54:43 -0500
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Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Dec 6, 2016 at 9:53 PM, Tom Lane <tgl@sss.pgh.pa.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


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