gnttab: miscellaneous fixes

- _GTF_* constants name bit positions, so binary arithmetic on them is
  wrong
- gnttab_clear_flag() cannot (on x86 and ia64 at least) simply use
  clear_bit(), as that may access more than the two bytes that are
  intended to be accessed

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
xen-unstable changeset:   24742:9fc810bb8145
xen-unstable date:        Thu Feb 09 16:39:16 2012 +0100

3 files changed, 25 insertions, 8 deletions

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 87aff17ac7..2bc86b76d1 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -356,7 +356,8 @@ static int _set_status_v2(domid_t  domid,
              (id != domid) ||
              (!readonly && (flags & GTF_readonly)) )
         {
-            gnttab_clear_flag(_GTF_reading | _GTF_writing, status);
+            gnttab_clear_flag(_GTF_writing, status);
+            gnttab_clear_flag(_GTF_reading, status);
             PIN_FAIL(done, GNTST_general_error,
                      "Unstable flags (%x) or dom (%d). (expected dom %d) "
                      "(r/w: %d)\n",
@@ -1668,14 +1669,14 @@ __release_grant_for_copy(
    under the domain's grant table lock. */
 /* Only safe on transitive grants.  Even then, note that we don't
    attempt to drop any pin on the referent grant. */
-static void __fixup_status_for_pin(struct active_grant_entry *act,
+static void __fixup_status_for_pin(const struct active_grant_entry *act,
                                    uint16_t *status)
 {
     if ( !(act->pin & GNTPIN_hstw_mask) )
-        *status &= ~_GTF_writing;
+        *status &= ~GTF_writing;
 
     if ( !(act->pin & GNTPIN_hstr_mask) )
-        *status &= ~_GTF_reading;
+        *status &= ~GTF_reading;
 }
 
 /* Grab a frame number from a grant entry and update the flags and pin
diff --git a/xen/include/asm-ia64/grant_table.h b/xen/include/asm-ia64/grant_table.h
index c540be36c5..27ee71b2af 100644
--- a/xen/include/asm-ia64/grant_table.h
+++ b/xen/include/asm-ia64/grant_table.h
@@ -5,6 +5,8 @@
 #ifndef __ASM_GRANT_TABLE_H__
 #define __ASM_GRANT_TABLE_H__
 
+#include <asm/intrinsics.h>
+
 #define INITIAL_NR_GRANT_FRAMES 1
 
 // for grant map/unmap
@@ -82,9 +84,19 @@ int guest_physmap_add_page(struct domain *d, unsigned long gpfn, unsigned long m
 
 #define gnttab_mark_dirty(d, f) ((void)f)
 
-static inline void gnttab_clear_flag(unsigned long nr, uint16_t *addr)
+static inline void gnttab_clear_flag(unsigned int nr, volatile uint16_t *st)
 {
-	clear_bit(nr, addr);
+	/*
+	 * Note that this cannot be clear_bit(), as the access must be
+	 * confined to the specified 2 bytes.
+	 */
+	uint16_t mask = ~(1 << nr), old;
+	CMPXCHG_BUGCHECK_DECL
+
+	do {
+		CMPXCHG_BUGCHECK(st);
+		old = *st;
+	} while (cmpxchg_rel(st, old, old & mask) != old);
 }
 
 #define gnttab_host_mapping_get_page_type(op, ld, rd)   \
diff --git a/xen/include/asm-x86/grant_table.h b/xen/include/asm-x86/grant_table.h
index ff029990a8..db8bfb46f2 100644
--- a/xen/include/asm-x86/grant_table.h
+++ b/xen/include/asm-x86/grant_table.h
@@ -48,9 +48,13 @@ int replace_grant_host_mapping(
 
 #define gnttab_mark_dirty(d, f) paging_mark_dirty((d), (f))
 
-static inline void gnttab_clear_flag(unsigned long nr, uint16_t *addr)
+static inline void gnttab_clear_flag(unsigned int nr, uint16_t *st)
 {
-    clear_bit(nr, (unsigned long *)addr);
+    /*
+     * Note that this cannot be clear_bit(), as the access must be
+     * confined to the specified 2 bytes.
+     */
+    asm volatile ("lock btrw %1,%0" : "=m" (*st) : "Ir" (nr), "m" (*st));
 }
 
 /* Foreign mappings of HHVM-guest pages do not modify the type count. */