About a change to the implementation of spin lock in 2.6.12 kernel.

[multisyncfe991]

Hi,

I found _spin_lock used a LOCK instruction to make the following operation 
"decb %0" atomic. As you know, LOCK instruction alone takes almost 70 clock 
cycles to finish and this add lots of cost to the _spin_lock. However 
_spin_unlock does not use this LOCK instruction and it uses "movb $1,%0" 
instead since 4-byte writes on 4-byte aligned addresses are atomic.

So I want rewrite the _spin_lock defined spinlock.h 
(/linux/include/asm-i386) as follows to reduce the overhead of _spin_lock 
and make it more efficient.
#define spin_lock_string \
       "\n1:\t" \
       "cmpb $0,%0\n\t" \
       "jle 2f\n\t" \
       "movb $0, %0\n\t" \
       "jmp 3f\n" \
       "2:\t" \
       "rep;nop\n\t" \
       "cmpb $0, %0\n\t" \
       "jle 2b\n\t" \
       "jmp 1b\n" \
       "3:\n\t"

Compared with the original version as follows, LOCK instruction is removed. 
I rebuilt the Intel e1000 Gigabit driver with this _spin_lock. There is 
about 2% throughput improvement.
#define spin_lock_string \
           "\n1:\t" \
           "lock ; decb %0\n\t" \
           "jns 3f\n" \
           "2:\t" \
           "rep;nop\n\t" \
           "cmpb $0,%0\n\t" \
           "jle 2b\n\t" \
           "jmp 1b\n" \
           "3:\n\t"

Do you think I can get a better performance if I dig further?

Any ideas will be greatly appreciated,

L.Y.
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