linux-kernel - Re: [PATCH v2] csky: fix csky_cmpxchg

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Message-ID: <CA+N+=zvG62nrmz=f1n1hCztu8k=KBg24_ZZKCWbn2Bhzd7cUdQ@mail.gmail.com>
Date: Mon, 21 Oct 2024 15:51:18 +0800
From: yang li <yang.li85200@...il.com>
To: Guo Ren <guoren@...nel.org>
Cc: linux-csky@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] csky: fix csky_cmpxchg_fixup not working

Hi Guo Ren:
In C language, your conclusion is correct, but in assembly language,
 global_symbol != &global_symbol
I did the following experiment:

liyang@...ang-virtual-machine:~/Desktop/test$ ls
main.c  test.s
liyang@...ang-virtual-machine:~/Desktop/test$ cat test.s

.globl test_symbol
test_symbol:
        nop
liyang@...ang-virtual-machine:~/Desktop/test$ cat main.c
#include <stdio.h>

extern unsigned long test_symbol;
int main(void)
{
        printf("test_symbol = 0x%lx\n",(unsigned long)test_symbol);
        printf("&test_symbol = 0x%lx\n",(unsigned long)&test_symbol);

        printf("main = 0x%lx\n",(unsigned long)main);
        printf("&main = 0x%lx\n",(unsigned long)&main);
}
liyang@...ang-virtual-machine:~/Desktop/test$ gcc main.c test.s --static -o test
liyang@...ang-virtual-machine:~/Desktop/test$ ls
main.c  test  test.s
liyang@...ang-virtual-machine:~/Desktop/test$ readelf test -s | grep test_symbol
   884: 000000000040170c     0 NOTYPE  GLOBAL DEFAULT    7 test_symbol
liyang@...ang-virtual-machine:~/Desktop/test$ readelf test -s | grep main -w
  1605: 0000000000401685   135 FUNC    GLOBAL DEFAULT    7 main
liyang@...ang-virtual-machine:~/Desktop/test$ ./test
test_symbol = 0x4b853001f0f90
&test_symbol = 0x40170c
main = 0x401685
&main = 0x401685

The above test can lead to the conclusion that:
Both c_symbol and &c_symbol represent the address of a symbol, but
&ASM_symbol represents the address of a symbol while ASM_symbol
 represents the opcode stored at that address.

On Thu, Oct 17, 2024 at 2:05 PM Guo Ren <guoren@...nel.org> wrote:
>
> On Wed, Oct 16, 2024 at 5:56 PM Yang Li <yang.li85200@...il.com> wrote:
> >
> > In the csky_cmpxchg_fixup function, it is incorrect to use the global
> >  variable csky_cmpxchg_stw to determine the address where the exception
> >  occurred.The global variable csky_cmpxchg_stw stores the opcode at the
> >  time of the exception, while &csky_cmpxchg_stw shows the address where
> >  the exception occurred.
> >
> > Signed-off-by: Yang Li <yang.li85200@...il.com>
> > ---
> > V1 -> V2:Eliminate compilation warnings
> >
> >  arch/csky/mm/fault.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/csky/mm/fault.c b/arch/csky/mm/fault.c
> > index a885518ce1dd..5226bc08c336 100644
> > --- a/arch/csky/mm/fault.c
> > +++ b/arch/csky/mm/fault.c
> > @@ -45,8 +45,8 @@ static inline void csky_cmpxchg_fixup(struct pt_regs *regs)
> >         if (trap_no(regs) != VEC_TLBMODIFIED)
> >                 return;
> >
> > -       if (instruction_pointer(regs) == csky_cmpxchg_stw)
> > -               instruction_pointer_set(regs, csky_cmpxchg_ldw);
> > +       if (instruction_pointer(regs) == (unsigned long)&csky_cmpxchg_stw)
> > +               instruction_pointer_set(regs, (unsigned long)&csky_cmpxchg_ldw);
> csky_cmpxchg_ldw(stw) is a label symbol, not a variable.
>
> arch/csky/kernel/atomic.S:
> GLOBAL(csky_cmpxchg_ldw)
> GLOBAL(csky_cmpxchg_stw)
>
> Your modification does not affect the ASM output.
>
> (gdb) p main
> $1 = {void (void)} 0x5fa <main>
> (gdb) p &main
> $2 = (void (*)(void)) 0x5fa <main>
>
> >         return;
> >  }
> >  #endif
> > --
> > 2.34.1
> >
>
>
> --
> Best Regards
>  Guo Ren