Messages in this thread |  | | Date | Thu, 11 Oct 2001 16:01:35 -0700 (PDT) | From | Davide Libenzi <> | Subject | Re: 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
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
|  |