On Thu, 14 Dec 2017 03:33:18 +0300 "Kirill A. Shutemov" [off-list ref] wrote:
On Wed, Dec 13, 2017 at 04:09:51PM -0800, Andrew Morton wrote:
quoted
quoted
@@ -181,6 +182,40 @@ static inline pmd_t native_pmdp_get_and_clear(pmd_t *pmdp)
#define native_pmdp_get_and_clear(xp) native_local_pmdp_get_and_clear(xp)
#endif
+#ifndef pmdp_establish
+#define pmdp_establish pmdp_establish
+static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
+ unsigned long address, pmd_t *pmdp, pmd_t pmd)
+{
+ pmd_t old;
+
+ /*
+ * If pmd has present bit cleared we can get away without expensive
+ * cmpxchg64: we can update pmdp half-by-half without racing with
+ * anybody.
+ */
+ if (!(pmd_val(pmd) & _PAGE_PRESENT)) {
+ union split_pmd old, new, *ptr;
+
+ ptr = (union split_pmd *)pmdp;
+
+ new.pmd = pmd;
+
+ /* xchg acts as a barrier before setting of the high bits */
+ old.pmd_low = xchg(&ptr->pmd_low, new.pmd_low);
+ old.pmd_high = ptr->pmd_high;
+ ptr->pmd_high = new.pmd_high;
+ return old.pmd;
+ }
+
+ {
+ old = *pmdp;
+ } while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd);
um, what happened here?
Ouch.. Yeah, we need 'do' here. :-/
Apparently, it's a valid C code that would run the body once and it worked for
me because I didn't hit the race condition.
So how the heck do we test this? Add an artificial delay on the other
side to open the race window?
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