From: "H . J . Lu" <redacted>
Subject: Re: PATCH: Fix ll/sc for mips
Date: Thu, 31 Jan 2002 14:41:00 -0800

On Thu, Jan 31, 2002 at 11:17:21PM +0100, Maciej W. Rozycki wrote:

quoted

On Thu, 31 Jan 2002, H . J . Lu wrote:

quoted

	(__compare_and_swap): Return 0 when failed to compare or swap.

[...]

quoted

	* sysdeps/mips/atomicity.h (compare_and_swap): Return 0 when
	failed to compare or swap.

 Looking at the i486 implementation these are not expected to fail. 
Unless I am missing something... 

Why can't it fail?

static inline char
__attribute__ ((unused))
compare_and_swap (volatile long int *p, long int oldval, long int newval)
{
  char ret;
  long int readval;

  __asm__ __volatile__ ("lock; cmpxchgl %3, %1; sete %0"
                        : "=q" (ret), "=m" (*p), "=a" (readval)
                        : "r" (newval), "1" (*p), "a" (oldval));
  return ret;
}

It will fail if *p is not same as oldval.

Please note that "sc" may fail even if nobody write the
variable. (See P.211 "8.4.2 Load-Linked/Sotre-Conditional" of "See 
MIPS RUN" for more detail.) 
So, after your patch applied, compare_and_swap() may fail, even if
*p is equal to oldval.

quoted

quoted

	* sysdeps/unix/sysv/linux/mips/sys/tas.h (_test_and_set): Fill
	the delay slot.

 What's the difference?  The code looks the same after changes.  Also you
forgot to indent instructions in delay slots, which worsens readability. 

Are we looking at the same file? Here is the patched version:

_EXTERN_INLINE int
_test_and_set (int *p, int v) __THROW 
{
  int r, t;

  __asm__ __volatile__
    (".set      push\n\t"
     ".set      noreorder\n"
     "1:\n\t"
     "ll        %0,%3\n\t"
     "beq       %0,%4,2f\n\t"
     "move      %1,%4\n\t"
     "sc        %1,%2\n\t"
     "beqz      %1,1b\n\t"
     "nop\n" 
     "2:\n\t"
     ".set      pop"    
     : "=&r" (r), "=&r" (t), "=m" (*p)
     : "m" (*p), "r" (v) 
     : "memory");

  return r;
}

Gas will fill delay slots. Same object codes will be produced, so I
think you don't have to do that by hand. 

---
Hiroyuki Machida
Sony Corp.