netdev - Re: [PATCH] net, llc: Avoid undefined behaviour in llc_conn_ac_inc_vr_by

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Date:	Sat, 25 Jun 2011 17:04:45 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	jj@...osbits.net
Cc:	acme@...stprotocols.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net, llc: Avoid undefined behaviour in
 llc_conn_ac_inc_vr_by_1()

From: Jesper Juhl <jj@...osbits.net>
Date: Sun, 26 Jun 2011 00:11:12 +0200 (CEST)

> Introduce a sequence point (;) between two writes to llc_sk(sk)->vR in
> net/llc/llc_c_ac.c:llc_conn_ac_inc_vr_by_1() so that the order in
> which the writes happen become well defined.
> 
> While the code may work fine now it may break at any time with a
> different compiler, a new version of current compiler or even just a
> different optimization level of the current compiler. Much better to
> clearly express what's intended in a way that guarantees the result.
> 
> Signed-off-by: Jesper Juhl <jj@...osbits.net>

How about fixing the macro so that it doesn't have side effects
like this?

That's much better than a 6 line (improperly formatted, BTW) comment
every time someone tried to use that macros with an lvalue that isn't
a local variable.
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