From: patrick <patrick@openbsd.org>
Date: Thu, 16 Aug 2018 15:36:04 +0000 (+0000)
Subject: Make pmap_allocate_asid() mpsafe.  Since between checking the ASID
X-Git-Url: http://artulab.com/gitweb/?a=commitdiff_plain;h=2a0627a87bdbe71676455b13fb5cb61220cf8a67;p=openbsd

Make pmap_allocate_asid() mpsafe.  Since between checking the ASID
table and setting the bits atomically another core can select the
same ASID as we did it currently would not be safe to run it without
the kernel lock.  This replaces the atomic_setbits_int(9) call with
atomic_cas_uint(9) where we can check that the table entry has not
been changed since we evaluted it.  Also modify pmap_free_asid() to
use the same concept.

ok kettenis@
---

diff --git a/sys/arch/arm64/arm64/pmap.c b/sys/arch/arm64/arm64/pmap.c
index af2294bead1..df734710e98 100644
--- a/sys/arch/arm64/arm64/pmap.c
+++ b/sys/arch/arm64/arm64/pmap.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: pmap.c,v 1.55 2018/08/15 20:18:31 kettenis Exp $ */
+/* $OpenBSD: pmap.c,v 1.56 2018/08/16 15:36:04 patrick Exp $ */
 /*
  * Copyright (c) 2008-2009,2014-2016 Dale Rahn <drahn@dalerahn.com>
  *
@@ -2174,20 +2174,27 @@ uint32_t pmap_asid[NUM_ASID / 32];
 void
 pmap_allocate_asid(pmap_t pm)
 {
+	uint32_t bits;
 	int asid, bit;
 
-	do {
-		asid = arc4random() & (NUM_ASID - 2);
-		bit = (asid & (32 - 1));
-	} while (asid == 0 || (pmap_asid[asid / 32] & (3U << bit)));
+	for (;;) {
+		do {
+			asid = arc4random() & (NUM_ASID - 2);
+			bit = (asid & (32 - 1));
+			bits = pmap_asid[asid / 32];
+		} while (asid == 0 || (bits & (3U << bit)));
 
-	atomic_setbits_int(&pmap_asid[asid / 32], 3U << bit);
+		if (atomic_cas_uint(&pmap_asid[asid / 32], bits,
+		    bits | (3U << bit)) == bits)
+			break;
+	}
 	pm->pm_asid = asid;
 }
 
 void
 pmap_free_asid(pmap_t pm)
 {
+	uint32_t bits;
 	int bit;
 
 	KASSERT(pm != curcpu()->ci_curpm);
@@ -2195,7 +2202,12 @@ pmap_free_asid(pmap_t pm)
 	cpu_tlb_flush_asid_all((uint64_t)(pm->pm_asid | ASID_USER) << 48);
 
 	bit = (pm->pm_asid & (32 - 1));
-	atomic_clearbits_int(&pmap_asid[pm->pm_asid / 32], 3U << bit);
+	for (;;) {
+		bits = pmap_asid[pm->pm_asid / 32];
+		if (atomic_cas_uint(&pmap_asid[pm->pm_asid / 32], bits,
+		    bits & ~(3U << bit)) == bits)
+			break;
+	}
 }
 
 void