[PATCHv6 3/5] gnttab: split grant table lock into table and maptrack locks

[David Vrabel <david.vrabel@citrix.com>]

From: Matt Wilson <msw@amazon.com>

The maptrack lock protects the maptrack state only.

Signed-off-by: Matt Wilson <msw@amazon.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
Subsequent changes make both these locks uncontented.  Is this patch
really necessary? -- dvrabel
---
 xen/common/grant_table.c      |   33 ++++++++++++++++++++++++++++-----
 xen/include/xen/grant_table.h |    7 ++++++-
 2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 3a555f9..a5470c8 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -264,10 +264,10 @@ static inline void
 put_maptrack_handle(
     struct grant_table *t, int handle)
 {
-    spin_lock(&t->lock);
+    spin_lock(&t->maptrack_lock);
     maptrack_entry(t, handle).ref = t->maptrack_head;
     t->maptrack_head = handle;
-    spin_unlock(&t->lock);
+    spin_unlock(&t->maptrack_lock);
 }
 
 static inline int
@@ -279,7 +279,7 @@ get_maptrack_handle(
     struct grant_mapping *new_mt;
     unsigned int          new_mt_limit, nr_frames;
 
-    spin_lock(&lgt->lock);
+    spin_lock(&lgt->maptrack_lock);
 
     while ( unlikely((handle = __get_maptrack_handle(lgt)) == -1) )
     {
@@ -308,12 +308,15 @@ get_maptrack_handle(
                  nr_frames + 1);
     }
 
-    spin_unlock(&lgt->lock);
+    spin_unlock(&lgt->maptrack_lock);
 
     return handle;
 }
 
-/* Number of grant table entries. Caller must hold d's grant table lock. */
+/* 
+ * Number of grant table entries. Caller must hold d's grant table
+ * read lock.
+ */
 static unsigned int nr_grant_entries(struct grant_table *gt)
 {
     ASSERT(gt->gt_version != 0);
@@ -531,6 +534,13 @@ static int grant_map_exists(const struct domain *ld,
     return -EINVAL;
 }
 
+/* 
+ * Count the number of mapped ro or rw entries tracked in the left
+ * grant table given a provided mfn provided by a foreign domain. 
+ *
+ * This function takes the maptrack lock from the left grant table and
+ * must be called with the right grant table's rwlock held.
+ */
 static void mapcount(
     struct grant_table *lgt, struct domain *rd, unsigned long mfn,
     unsigned int *wrc, unsigned int *rdc)
@@ -540,6 +550,16 @@ static void mapcount(
 
     *wrc = *rdc = 0;
 
+    /* 
+     * While taking the local maptrack spinlock prevents any mapping
+     * changes, the remote active entries could be changing while we
+     * are counting. The caller has to hold the grant table lock, or
+     * some other mechanism should be used to prevent concurrent
+     * changes during this operation.
+     */
+    ASSERT(spin_is_locked(&rd->grant_table->lock));
+    spin_lock(&lgt->maptrack_lock);
+
     for ( handle = 0; handle < lgt->maptrack_limit; handle++ )
     {
         struct active_grant_entry *act;
@@ -552,6 +572,8 @@ static void mapcount(
         if ( act->frame == mfn )
             (map->flags & GNTMAP_readonly) ? (*rdc)++ : (*wrc)++;
     }
+
+    spin_unlock(&lgt->maptrack_lock);
 }
 
 /*
@@ -2980,6 +3002,7 @@ grant_table_create(
 
     /* Simple stuff. */
     spin_lock_init(&t->lock);
+    spin_lock_init(&t->maptrack_lock);
     t->nr_grant_frames = INITIAL_NR_GRANT_FRAMES;
 
     /* Active grant table. */
diff --git a/xen/include/xen/grant_table.h b/xen/include/xen/grant_table.h
index 32f5786..8ff1883 100644
--- a/xen/include/xen/grant_table.h
+++ b/xen/include/xen/grant_table.h
@@ -82,7 +82,12 @@ struct grant_table {
     struct grant_mapping **maptrack;
     unsigned int          maptrack_head;
     unsigned int          maptrack_limit;
-    /* Lock protecting updates to active and shared grant tables. */
+    /* Lock protecting the maptrack page list, head, and limit */
+    spinlock_t            maptrack_lock;
+    /* 
+     * Lock protecting updates to grant table state (version, active
+     * entry list, etc.)
+     */
     spinlock_t            lock;
     /* The defined versions are 1 and 2.  Set to 0 if we don't know
        what version to use yet. */
-- 
1.7.10.4