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SubjectRe: Question and patch about spinlocks (x86)
On Thu, 11 Oct 2001, Mark Zealey wrote:

> Just looking through at the spinlock assembly I noticed a few things which I
> think are bugs:
>
> "js 2f\n" \
> ".section .text.lock,\"ax\"\n" \
> "2:\t" \
> "cmpb $0,%0\n\t" \
> "rep;nop\n\t" \
> "jle 2b\n\t" \
> "jmp 1b\n" \
> ".previous"
>
> We do the cmp loop as a 'soft' check, as the lock operand locks the whole system
> bus, stopping the system for a while (as much as 70 cycles, I believe). However,
> I don't understand why it was put before the 'rep; nop' which just sets the
> processor to wait for a bit. Surely it would be better to test *after* we have
> waited, as then we have a better chance of it being correct.

The effect of the rep-nop is not to wait but to slow down the cpu to the
speed of the memory bus.
This to not overload ( due pipeline prefetch ) the memory controller with
requests that 1) will be useless coz the watched memory location can
change only at the membus speed 2) will have a big cost on loop exit due
the invalidation of a number>1 requests issued on the memory controller.
Beside this i kindly agree to move the pause before the cmp.
There should be a valid reason to not have followed the intel scheme but i
don't know why.



- Davide


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