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To: attilio@FreeBSD.org
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Tue, 18 Sep 2012 16:44:08 +0100."
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Date: Tue, 18 Sep 2012 15:53:09 +0000
Message-ID: <22286.1347983589@critter.freebsd.dk>
Cc: arch@FreeBSD.org
Subject: Re: Aliasing issue with TAILQ on ppc64 ?
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In message <CAJ-FndCsf2Xsn=1ioHyr_tn3-yAFOE7E9-wrjp4rcQJajhZvpg@mail.gmail.com>
, Attilio Rao writes:

>The only way I can see this
>code is safe is, infact, to lock it with proper locks around the
>operations.

This is not about locking: at the time where this croaks there is
only one thread.

The problem is that:

	// Empty, freshly initialized ban_head

	b = valid_ban_object();
	TAILQ_INSERT_HEAD(&ban_head, b, list);

	be = TAILQ_LAST(&ban_head, banhead_s);

Causes a sig#11 in TAILQ_LAST().

I belive it is a NULL dereference, and I belive it happens
because the compiler overoptimizes TAILQ_{LAST|PREV}()

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.