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

From: Matt Wilson <msw@xxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxxxx
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Anthony Liguori <aliguori@xxxxxxxxxx>
Signed-off-by: Matt Wilson <msw@xxxxxxxxxx>

* 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;
     op->map = &maptrack_entry(lgt, op->handle);
-    spin_lock(&lgt->lock);
     if ( unlikely(!op->map->flags) )

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