On 05/24/2013 10:13 AM, Chen Gang wrote:

On 05/23/2013 10:10 PM, Geert Uytterhoeven wrote:

quoted

On Thu, May 23, 2013 at 2:50 PM, Russell King - ARM Linux
[off-list ref] wrote:

quoted

quoted

On Thu, May 23, 2013 at 02:09:02PM +0200, Arnd Bergmann wrote:

quoted

quoted

On Thursday 23 May 2013, Russell King - ARM Linux wrote:

quoted

quoted

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