powerpc/mm: Check for matching hpte without taking hpte lock

With smaller hash page table config, we would end up in situation
where we would be replacing hash page table slot frequently. In
such config, we will find the hpte to be not matching, and we
can do that check without holding the hpte lock. We need to
recheck the hpte again after holding lock.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/hash_native_64.c

@@ -294,8 +294,6 @@ static long native_hpte_updatepp(unsigned long slot, unsigned long newpp,
 	DBG_LOW("    update(vpn=%016lx, avpnv=%016lx, group=%lx, newpp=%lx)",
 		vpn, want_v & HPTE_V_AVPN, slot, newpp);
 
-	native_lock_hpte(hptep);
-
 	hpte_v = be64_to_cpu(hptep->v);
 	/*
 	 * We need to invalidate the TLB always because hpte_remove doesn't do
@@ -307,17 +305,25 @@ static long native_hpte_updatepp(unsigned long slot, unsigned long newpp,
 	if (!HPTE_V_COMPARE(hpte_v, want_v) || !(hpte_v & HPTE_V_VALID)) {
 		DBG_LOW(" -> miss\n");
 		ret = -1;
+	} else {
+		native_lock_hpte(hptep);
+		/* recheck with locks held */
+		hpte_v = be64_to_cpu(hptep->v);
+		if (unlikely(!HPTE_V_COMPARE(hpte_v, want_v) ||
+			     !(hpte_v & HPTE_V_VALID))) {
+			ret = -1;
 		} else {
 			DBG_LOW(" -> hit\n");
 			/* Update the HPTE */
-		hptep->r = cpu_to_be64((be64_to_cpu(hptep->r) & ~(HPTE_R_PP | HPTE_R_N)) |
-			(newpp & (HPTE_R_PP | HPTE_R_N | HPTE_R_C)));
+			hptep->r = cpu_to_be64((be64_to_cpu(hptep->r) &
+						~(HPTE_R_PP | HPTE_R_N)) |
+					       (newpp & (HPTE_R_PP | HPTE_R_N |
+							 HPTE_R_C)));
 		}
 		native_unlock_hpte(hptep);
-
+	}
 	/* Ensure it is out of the tlb too. */
 	tlbie(vpn, bpsize, apsize, ssize, local);
-
 	return ret;
 }