On Fri, 14 Oct 2005, Chuck Ebbert wrote:
> Attempt to acquire spinlock sooner after spinning and then noticing
> it has become available. Also adds a slight delay before testing the
> spinlock again when it's not available, reducing bus traffic.
>
> This change makes spinlocks fairer in the case where the owner drops
> the lock and then immediately tries to take it again.
>
> Signed-Off-By: Chuck Ebbert <[email protected]>
> ---
>
> include/asm-i386/spinlock.h | 4 ++--
> 1 files changed, 2 insertions(+), 2 deletions(-)
>
> --- 2.6.14-rc4a.orig/include/asm-i386/spinlock.h
> +++ 2.6.14-rc4a/include/asm-i386/spinlock.h
> @@ -28,8 +28,8 @@
> "2:\t" \
> "rep;nop\n\t" \
> "cmpb $0,%0\n\t" \
> - "jle 2b\n\t" \
> - "jmp 1b\n" \
> + "jg 1b\n\t" \
> + "jmp 2b\n" \
> "3:\n\t"
>
> #define __raw_spin_lock_string_flags \
> -
I just noticed in linux-2.6.13.4 that the spinlocks are wrong.
You are trying to optimize a path that should have been granted
in the first place:
Somehow, these spin-locks got all screwed up.
Given: nobody has the lock. The lock variable is 0.
#define spin_lock_string \
"\n1:\t" \
"lock ; decb %0\n\t" \
"jns 3f\n" \ # This should have been 'js'
"2:\t" \
"rep;nop\n\t" \
"cmpb $0,%0\n\t" \
"jle 2b\n\t" \
"jmp 1b\n" \
"3:\n\t" # Got the lock, exit loop
The spin-locks should have been:
#define spin_lock_string \
"\n1:\t" \
"lock ; decb %0\n\t" \ # Bump the lock variable
"js 2f\n\t" \ # Good, we got the lock
"lock ; incb %0\n\t" \ # Release your attempt __important__
"rep ; nop \n\t" \ # pause
"jmp 1b\n" \ # Try again
"2:\n" # Exit the lock
Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.46 BogoMips).
Warning : 98.36% of all statistics are fiction.
.
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