linux-kernel - Re: [PATCH 55/74] lto, workaround: Add workaround for initcall reordering

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Date:	Mon, 20 Aug 2012 12:00:44 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	<x86@...nel.org>, <akpm@...ux-foundation.org>,
	<ak@...ux.intel.com>, <mmarek@...e.cz>,
	<linux-kbuild@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 55/74] lto, workaround: Add workaround for initcall
 reordering

>>> On 19.08.12 at 17:01, Andi Kleen <andi@...stfloor.org> wrote:
> On Sun, Aug 19, 2012 at 09:46:04AM +0100, Jan Beulich wrote:
>> >>> Andi Kleen <andi@...stfloor.org> 08/19/12 5:05 AM >>>
>> >Work around a LTO gcc problem: when there is no reference to a variable
>> >in a module it will be moved to the end of the program. This causes
>> >reordering of initcalls which the kernel does not like.
>> >Add a dummy reference function to avoid this. The function is
>> >deleted by the linker.
>> 
>> This is not even true on x86, not to speak of generally.
> 
> Why is it not true ? 
> 
> __initcall is only defined for !MODULE and there __exit discards.

__exit, on x86 and perhaps other arches, causes the code
to be discarded at runtime only.

>> >+#ifdef CONFIG_LTO
>> >+/* Work around a LTO gcc problem: when there is no reference to a variable
>> >+ * in a module it will be moved to the end of the program. This causes
>> >+ * reordering of initcalls which the kernel does not like.
>> >+ * Add a dummy reference function to avoid this. The function is 
>> >+ * deleted by the linker.
>> >+ */
>> >+#define LTO_REFERENCE_INITCALL(x) \
>> >+    ; /* yes this is needed */            \
>> >+    static __used __exit void *reference_##x(void)     \
>> 
>> Why not put it into e.g. section .discard.text? That could be expected to be
>> discarded by the linker without being arch dependent, as long as all arches
>> use DISCARDS in their linker script.
> 
> 
> That's what __exit does, doesn't it?

No - see above. Using .discard.* enforces the discarding at link
time.

Jan

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