Re: [PATCH] avoid unnecessary cache flushes

Richard Henderson
2001-09-03T20:41:33Z

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~