linux-kernel - Re: [PATCH] /dev/mem gcc weak function workaround

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Message-Id: <20080429.212833.192304794.davem@davemloft.net>
Date:	Tue, 29 Apr 2008 21:28:33 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	venkatesh.pallipadi@...el.com
Cc:	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] /dev/mem gcc weak function workaround

From: Venki Pallipadi <venkatesh.pallipadi@...el.com>
Date: Tue, 29 Apr 2008 18:31:09 -0700

> Some flavors of gcc 4.1.0 and 4.1.1 seems to have trouble understanding
> weak function definitions. Calls to function from the same file where it is
> defined as weak _may_ get inlined, even when there is a non-weak definition of
> the function elsewhere. I tried using attribute 'noinline' which does not
> seem to help either.
> 
> One workaround for this is to have weak functions defined in different
> file as below. Other possible way is to not use weak functions and go back
> to ifdef logic.
> 
> There are few other usages in kernel that seem to depend on weak (and noinline)
> working correctly, which can also potentially break and needs such workarounds.
> Example -
> mach_reboot_fixups() in arch/x86/kernel/reboot.c is one such call which
> is getting inlined with a flavor of gcc 4.1.1.
> 
> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
> Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>

This sounds like a bug.  And if gcc does multi-file compilation it
can in theory do the same mistake even if you move it to another
file.

We need something more bulletproof here.

Also, we have a macro for marking things weak "__weak" which should
be used here.
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