[RFC PATCH 1/2] gnttab: lock the local grant table earlier in __gnttab_unmap_common()

[Matt Wilson <msw@linux.com>]

From: Matt Wilson <msw@amazon.com>

Luckily today maptrack_limit never shrinks. But if at some point in
the future this were to change, checking maptrack_limit without
holding the grant table spinlock would no longer be safe.

Cc: xen-devel@lists.xenproject.org
Cc: Keir Fraser <keir@xen.org>
Cc: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Anthony Liguori <aliguori@amazon.com>
Signed-off-by: Matt Wilson <msw@amazon.com>

---
v1->v2:
* updated summary to use "local" instead of "left"
---
 xen/common/grant_table.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 21c6a14..ef10ff4 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -842,15 +842,16 @@ __gnttab_unmap_common(
 
     op->frame = (unsigned long)(op->dev_bus_addr >> PAGE_SHIFT);
 
+    spin_lock(&lgt->lock);
     if ( unlikely(op->handle >= lgt->maptrack_limit) )
     {
+        spin_unlock(&lgt->lock);
         gdprintk(XENLOG_INFO, "Bad handle (%d).\n", op->handle);
         op->status = GNTST_bad_handle;
         return;
     }
 
     op->map = &maptrack_entry(lgt, op->handle);
-    spin_lock(&lgt->lock);
 
     if ( unlikely(!op->map->flags) )
     {
-- 
1.7.9.5