From: Andrew Morton <akpm@linux-foundation.org>
To: David Daney <ddaney.cavm@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
David Rientjes <rientjes@google.com>,
linux-mips@linux-mips.org, ralf@linux-mips.org,
linux-kernel@vger.kernel.org,
David Daney <david.daney@cavium.com>,
linux-arch@vger.kernel.org, Robin Holt <holt@sgi.com>
Subject: Re: [patch] hugetlb: remove dummy definitions of HPAGE_MASK and HPAGE_SIZE
Date: Mon, 21 Nov 2011 15:53:31 -0800 [thread overview]
Message-ID: <20111121155331.a1726ffe.akpm@linux-foundation.org> (raw)
In-Reply-To: <4ECAE314.9060209@gmail.com>
On Mon, 21 Nov 2011 15:47:32 -0800
David Daney <ddaney.cavm@gmail.com> wrote:
> Just to expand on this lovely topic...
>
> On 11/21/2011 02:43 PM, Linus Torvalds wrote:
> > On Mon, Nov 21, 2011 at 2:23 PM, David Daney<ddaney.cavm@gmail.com> wrote:
> >>
> >> This whole comment strikes me as somewhat dishonest, as at the time David
> >> Rientjes wrote it, he knew that there were dependencies on these symbols in
> >> the linux-next tree.
> >>
> >> Now we can add these:
> >> +#define HPAGE_SHIFT ({ BUG(); 0; })
> >> +#define HPAGE_SIZE ({ BUG(); 0; })
> >> +#define HPAGE_MASK ({ BUG(); 0; })
> >
> > Hell no.
> >
> > We don't do run-time BUG() things. No way, no how.
> >
>
> These symbols are on dead code paths, so they are eliminated by the
> compiler's Dead Code Elimination (DCE) optimizations, and the BUG() code
> never gets emitted to the final executable.
>
> I agree that it is not the best thing to do, but given the current state
> of the art in build bug macros, it is the best we could have done.
>
> What I think we need instead, and for which I will send a patch soon, is
> something like this:
>
> extern void you_are_screwed() __attribute__ ((error("BUILD_BUG_ON_USED
> failed")));
> #define BUILD_BUG_ON_USED() you_are_screwed()
> #define HPAGE_SHIFT ({ BUILD_BUG_ON_USED(); 0; })
>
> This allows us to use the symbols in straight line C code without a ton
> of ugly #ifdefery, but give us build time error checking.
The way we usually handle that is to emit a call to a
this_function_does_not_exist(), so it fails at link time.
I guess we should do that to all those follow_hugetlb_page() and
friends.
gcc has had issues at times where it incorrectly emits references to
this_function_does_not_exist(), but that hasn't happened in a couple of
years as far as I know.
next prev parent reply other threads:[~2011-11-21 23:53 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-11-17 21:57 [PATCH v2 0/2] Dummy HPAGE_* constants for !CONFIG_HUGETLB_PAGE David Daney
2011-11-17 21:57 ` [PATCH v2 1/2] hugetlb: Provide a default HPAGE_SHIFT if !CONFIG_HUGETLB_PAGE David Daney
2011-11-17 21:57 ` [PATCH v2 2/2] hugetlb: Provide safer dummy values for HPAGE_MASK and HPAGE_SIZE David Daney
2011-11-17 23:28 ` Andrew Morton
2011-11-17 23:38 ` David Daney
2011-11-18 8:56 ` Sergei Shtylyov
2011-11-18 17:14 ` David Daney
2011-11-17 23:22 ` [PATCH v2 0/2] Dummy HPAGE_* constants for !CONFIG_HUGETLB_PAGE David Rientjes
2011-11-17 23:22 ` [patch] hugetlb: remove dummy definitions of HPAGE_MASK and HPAGE_SIZE David Rientjes
2011-11-17 23:35 ` Andrew Morton
2011-11-17 23:44 ` David Rientjes
2011-11-17 23:52 ` David Daney
2011-11-17 23:57 ` Andrew Morton
2011-11-17 23:57 ` David Rientjes
2011-11-21 22:23 ` David Daney
2011-11-21 22:43 ` Linus Torvalds
2011-11-21 23:23 ` David Daney
2011-11-21 23:43 ` Linus Torvalds
2011-11-21 23:50 ` Andrew Morton
2011-11-22 20:41 ` Andi Kleen
2011-11-21 23:47 ` David Daney
2011-11-21 23:53 ` Andrew Morton [this message]
2011-11-22 0:37 ` Linus Torvalds
2011-11-22 0:48 ` David Daney
2011-11-22 0:55 ` Linus Torvalds
2011-11-21 22:48 ` David Rientjes
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