Re: [PATCH] avoid unnecessary cache flushes

[Richard Henderson]

On Mon, Sep 03, 2001 at 01:27:02PM -0700, David Mosberger wrote:
>   >> +		if (!test_bit(PG_arch_1, &page->flags)) {
>   >> +			__flush_dcache_icache((unsigned long)kmap(page));
>   >> +			kunmap(page);
>   >> +			set_bit(PG_arch_1, &page->flags);
> 
>   Richard> Race.  Use test_and_set_bit.
> 
> Nope, the old code is correct: you must turn on PG_arch_1 *after*
> flushing the cache.  Yes, you might sometimes double flush, but that's
> both safe and rare.

You can get a missed flush from

	bit == 0
	flush cache

				modify page
				bit = 0

	bit = 1

unless this is protected from some outer lock of which 
I am not aware.

I do see your point about the early set though.

r~