[PATCH v2] cris: arch-v10: kgdb: Add volatile for static variable is_dyn_brkp

[PATCH v2] cris: arch-v10: kgdb: Add volatile for static variable is_dyn_brkp

[Chen Gang]

Within one C file, current gcc can optimize the global static variables
according to the C code, but it will skip assembly code -- it will pass
them to gas directly.

if the static variable is used between C code and assembly code in one C
file (e.g. is_dyn_brkp in kgdb.c), it needs volatile to let gcc know it
should not be optimized, or it may cause issue.

The related error in this case:

    LD      init/built-in.o
  arch/cris/arch-v10/kernel/built-in.o: In function `kgdb_handle_breakpoint':
  (.text+0x2aca): undefined reference to `is_dyn_brkp'
  arch/cris/arch-v10/kernel/built-in.o: In function `is_static':
  kgdb.c:(.text+0x2ada): undefined reference to `is_dyn_brkp'


Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
---
 arch/cris/arch-v10/kernel/kgdb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/cris/arch-v10/kernel/kgdb.c b/arch/cris/arch-v10/kernel/kgdb.c
index 5b61335..8faddd3 100644
--- a/arch/cris/arch-v10/kernel/kgdb.c
+++ b/arch/cris/arch-v10/kernel/kgdb.c
@@ -351,7 +351,7 @@ char internal_stack[INTERNAL_STACK_SIZE];
    breakpoint to be handled. A static breakpoint uses the content of register
    BRP as it is whereas a dynamic breakpoint requires subtraction with 2
    in order to execute the instruction. The first breakpoint is static. */
-static unsigned char is_dyn_brkp = 0;
+static volatile unsigned char is_dyn_brkp;
 
 /********************************* String library ****************************/
 /* Single-step over library functions creates trap loops. */
-- 
1.9.3

Re: [PATCH v2] cris: arch-v10: kgdb: Add volatile for static variable is_dyn_brkp

[Hans-Peter Nilsson]

> From: Chen Gang <xili_gchen_5257@hotmail.com>
> Date: Fri, 10 Jul 2015 23:50:07 +0200

> Within one C file, current gcc can optimize the global static variables
> according to the C code, but it will skip assembly code -- it will pass
> them to gas directly.
> 
> if the static variable is used between C code and assembly code in one C
> file (e.g. is_dyn_brkp in kgdb.c), it needs volatile to let gcc know it
> should not be optimized, or it may cause issue.

In this case it's *mostly* a matter of taste but please avoid
using volatile as a hammer when there are other tools available.

> -static unsigned char is_dyn_brkp = 0;
> +static volatile unsigned char is_dyn_brkp;

Please instead use "__used", i.e.
+static unsigned char __used is_dyn_brkp = 0;

brgds, H-P

Re: [PATCH v2] cris: arch-v10: kgdb: Add volatile for static variable is_dyn_brkp

[Chen Gang]

On 8/4/15 23:01, Hans-Peter Nilsson wrote:
>> From: Chen Gang <xili_gchen_5257@hotmail.com>
>> Date: Fri, 10 Jul 2015 23:50:07 +0200
> 
>> Within one C file, current gcc can optimize the global static variables
>> according to the C code, but it will skip assembly code -- it will pass
>> them to gas directly.
>>
>> if the static variable is used between C code and assembly code in one C
>> file (e.g. is_dyn_brkp in kgdb.c), it needs volatile to let gcc know it
>> should not be optimized, or it may cause issue.
> 
> In this case it's *mostly* a matter of taste but please avoid
> using volatile as a hammer when there are other tools available.
> 
>> -static unsigned char is_dyn_brkp = 0;
>> +static volatile unsigned char is_dyn_brkp;
> 
> Please instead use "__used", i.e.
> +static unsigned char __used is_dyn_brkp = 0;
> 

OK, thanks. I shall send patch v3 for it, next.

Thanks.
-- 
Chen Gang

Open, share, and attitude like air, water, and life which God blessed