Messages in this thread |  | | From | "Christian Ehrhardt" <> | Date | Tue, 27 Aug 2002 11:22:19 +0200 | Subject | Re: MM patches against 2.5.31 |
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On Mon, Aug 26, 2002 at 12:24:50PM -0700, Andrew Morton wrote: > The flaw is in doing the put_page_testzero() outside of any locking > which would prevent other CPUs from finding and "rescuing" the zero-recount > page. > > CPUA: > if (put_page_testzero()) { > /* Here's the window */ > spin_lock(lru_lock); > list_del(page->lru); > > CPUB: > > spin_lock(lru_lock); > page = list_entry(lru); > page_cache_get(page); /* If this goes from 0->1, we die */ > ... > page_cache_release(page); /* double free */
So what we want CPUB do instead is
spin_lock(lru_lock); page = list_entry(lru)
START ATOMIC page_cache_get(page); res = (page_count (page) == 1) END ATOMIC
if (res) { atomic_dec (&page->count); continue; /* with next page */ } ... page_cache_release (page);
I.e. we want to detect _atomically_ that we just raised the page count from zero to one. My patch actually has a solution that implements the needed atomic operation above by means of the atomic functions that we currently have on all archs (it's called get_page_testzero and should probably called get_page_testone). The more I think about this the more I think this is the way to go.
regards Christian
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