From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: pgsql-hackers@postgresql.org, Daniel Gustafsson <daniel@yesql.se>
Subject: Re: Bring atomic flag fallback up to snuff
In-reply-to: <20180406233854.uni2h3mbnveczl32@alap3.anarazel.de>
References: <20180406233854.uni2h3mbnveczl32@alap3.anarazel.de>
Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Fri, 06 Apr 2018 16:38:54 -0700"
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Date: Sat, 07 Apr 2018 14:07:05 -0400
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Andres Freund <andres@anarazel.de> writes:
> As Daniel pointed out in:
> https://postgr.es/m/FB948276-7B32-4B77-83E6-D00167F8EEB4@yesql.se the
> pg_atomic_flag fallback implementation is broken.  That has gone
> unnoticed because the fallback implementation wasn't testable until now:
> ...
> The attached fixes the bug and removes the edge-cases by storing a value
> separate from the semaphore. I should have done that from the start.
> This is an ABI break, but given the fallback didn't work at all, I don't
> think that's a problem for backporting.

> Fix attached. Comments?

pademelon says it's wrong.

2018-04-07 13:39:34.982 EDT [1197:89] pg_regress/lock LOG:  statement: SEL=
ECT test_atomic_ops();
TRAP: UnalignedPointer("(((uintptr_t) ((uintptr_t)(ptr)) + ((sizeof(*ptr))=
 - 1)) & ~((uintptr_t) ((sizeof(*ptr)) - 1))) !=3D (uintptr_t)(ptr)", File=
: "../../../src/include/port/atomics.h", Line: 177)


			regards, tom lane