On 05/24/2013 10:13 AM, Chen Gang wrote:
> On 05/23/2013 10:10 PM, Geert Uytterhoeven wrote:
>> On Thu, May 23, 2013 at 2:50 PM, Russell King - ARM Linux
>> <li...@arm.linux.org.uk> wrote:
>>>> On Thu, May 23, 2013 at 02:09:02PM +0200, Arnd Bergmann wrote:
>>>>>> On Thursday 23 May 2013, Russell King - ARM Linux wrote:
>>>>>>>> This is the problem you guys are missing - unreachable() means "we lose
>>>>>>>> control of the CPU at this point".
>>>>>>
>>>>>> I'm absolutely aware of this. Again, the current behaviour of doing
>>>>>> nothing
>>>>>> at all isn't very different from undefined behavior when you get when you
>>>>>> get to the end of a function returning a pointer without a "return"
>>>>>> statement,
>>>>>> or when you return from a function that has determined that it is not
>>>>>> safe
>>>>>> to continue.
>>>>
>>>> Running off the end of a function like that is a different kettle of fish.
>>>> The execution path is still as the compiler intends - what isn't is that
>>>> the data returned is likely to be random trash.
>>>>
>>>> That's _quite_ different from the CPU starting to execute the contents
>>>> of a literal data pool.
>> I agree it's best to e.g. trap and reboot.
>
In fact: if enable CONFIG_BUG, but not enable HAVE_ARCH_BUG, the
default implementation is:
47 #ifndef HAVE_ARCH_BUG
48 #define BUG() do { \
49 printk("BUG: failure at %s:%d/%s()!\n", __FILE__, __LINE__,
__func__); \
50 panic("BUG!"); \
51 } while (0)
52 #endif
So if we delete CONFIG_BUG, the default implementation will be almost
like panic(), and in panic() itself, also calls printk() !!
So...
:-)
> After read the arch/*/include/asm/bug.h,
>
> It seems panic() is not suitable for NOMMU platforms (only m68k use it,
> also need CONFIG_BUG and CONFIG_SUN3 enabled).
>
> And unreachable() is need followed with an asm inline instruction (arm,
> x86, powerpc mips...).
>
> And __builtin_trap() is "the mechanism used may vary from release to
> release so should not rely on any particular implementation" (ref to
> "http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html";, used by m68k,
> sparc, ia64).
>
> I can not find any *trap*() and *unreachable*() in "include/asm-generic/"
>
> I can not find any suitable implementation which 'generic' enough to add
> in "include/asm-generic/" (and in fact, CONFIG_BUG itself is not
> 'generic' enough to be in "include/asm-generic/").
>
>
> At last, I still suggest to delete CONFIG_BUG, so most of architectures
> can skip this issue firstly.
>
> Then for specific architectures, also can get 3 benefits:
>
> a. the related maintainers can implement it as their own willing (not
> need discus it with another platform maintainers again);
>
> b. the related maintainers can free use the platform specific features
> (which can not be used in "include/asm-generic/");
>
> c. the related maintainers are more familiar their own architectures
> demands and requirements.
>
>
>
> ----------- arch/m68k/include/asm/bug.h --------------------------------
>
> 1 #ifndef _M68K_BUG_H
> 2 #define _M68K_BUG_H
> 3
> 4 #ifdef CONFIG_MMU
> 5 #ifdef CONFIG_BUG
> 6 #ifdef CONFIG_DEBUG_BUGVERBOSE
> 7 #ifndef CONFIG_SUN3
> 8 #define BUG() do { \
> 9 printk("kernel BUG at %s:%d!\n", __FILE__, __LINE__); \
> 10 __builtin_trap(); \
> 11 } while (0)
> 12 #else
> 13 #define BUG() do { \
> 14 printk("kernel BUG at %s:%d!\n", __FILE__, __LINE__); \
> 15 panic("BUG!"); \
> 16 } while (0)
> 17 #endif
> 18 #else
> 19 #define BUG() do { \
> 20 __builtin_trap(); \
> 21 } while (0)
> 22 #endif
> 23
> 24 #define HAVE_ARCH_BUG
> 25 #endif
> 26 #endif /* CONFIG_MMU */
> 27
> 28 #include <asm-generic/bug.h>
> 29
> 30 #endif
>
>
>
>
> Thanks.
>
--
Chen Gang
Asianux Corporation
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