linux-kernel - Re: [PATCH] rcu: avoid checking for constant

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Date:	Thu, 12 Jan 2012 17:08:21 +0100 (CET)
From:	Jan Engelhardt <jengelh@...ozas.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Josh Triplett <josh@...htriplett.org>, laijs@...fujitsu.com,
	manfred@...orfullife.com, dhowells@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: avoid checking for constant

On Thursday 2012-01-12 16:29, Paul E. McKenney wrote:

>On Thu, Jan 12, 2012 at 11:57:10AM +0100, Jan Engelhardt wrote:
>> On Thursday 2012-01-12 11:54, Eric Dumazet wrote:
>> 
>> >Le jeudi 12 janvier 2012 à 11:34 +0100, Jan Engelhardt a écrit :
>> >
>> >> When compiling kernel or module code with -O0, "offset" is no longer
>> >> considered a constant, and therefore always triggers the build error
>> >> that BUILD_BUG_ON is defined to yield.
>> >
>> >Why compiling kernel with -O0 is even considered ?
>> 
>> The compile error was observed when trying to compile an out-of-tree 
>> module with -O0.
>
>Hmmm...  Why not have a glue .c file in your out-of-tree module
>that contains function like[...]

Maybe, maybe naah. The kernel relying on gcc's -O2 mode to do constant 
value analysis through and into inline functions does not sound too 
great. The compiler could change on the next moonphase. In fact, one 
can't use clang -O2 because of this. With my patch, one can again use 
clang -O2 [with respect to kfree_rcu].
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