On 1/26/2021 1:40 AM, Peter Zijlstra wrote:
On Tue, Jan 26, 2021 at 09:46:36AM +0100, Peter Zijlstra wrote:
On Mon, Jan 25, 2021 at 07:27:09PM +0100, Borislav Petkov wrote:
+ pte_t old_pte, new_pte;
+
+ do {
+ old_pte = READ_ONCE(*ptep);
+ new_pte = pte_wrprotect(old_pte);
Maybe I'm missing something but those two can happen outside of the
loop, no? Or is *ptep somehow changing concurrently while the loop is
doing the CMPXCHG and you need to recreate it each time?
IOW, you can generate upfront and do the empty loop...
+
+ } while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
+
+ return;
+ }
Empty loop would be wrong, but that wants to be written like:
old_pte = READ_ONCE(*ptep);
do {
new_pte = pte_wrprotect(old_pte);
} while (try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
! went missing, too early, moar wake-up juice.
Since try_cmpxchg() will update old_pte on failure.
Thanks Peter! I will fix that.
--
Yu-cheng
