Fix up the synchronisation around grant table map track handles.

At present, we're not doing any at all, so if a domain e.g. tries to
do two map operations at the same time from different vcpus then you
could end up with both operations getting back the same maptrack
handle.

Fix this problem by just shoving an enormous lock around grant table
operations.  This is unlikely to be heavily contended, because netback
and blkback both restrict themselves to mapping on a single vcpu at a
time (globally for netback, and per-device for blkback), and most of
the interesting bits are already protected by the remote domain's
grant table lock anyway.

The unconteded acquisition cost might be significant for some
workloads.  If that were the case, it might be worth only acquiring
the lock only for multi-vcpu domains, since we only manipulate the
maptrack table in the context of one of the domain's vcpus.  I've not
done that optimisation here, because I didn't want to think about what
would happen if e.g. a cpu got hot-unplugged from a domain while it
was performing a map operation.

Signed-off-by: Steven Smith <steven.smith@citrix.com>

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 7bbc05d896724a5cdf9680a9631e26ffcfc1e7e2..58011836dbf99ecad654ed6625fc13d5fa781a83 100644 (file)
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -111,6 +111,26 @@ static unsigned inline int max_nr_maptrack_frames(void)
 #define active_entry(t, e) \
     ((t)->active[(e)/ACGNT_PER_PAGE][(e)%ACGNT_PER_PAGE])
 
+/* Technically, we only really need to acquire the lock for SMP
+   guests, because you only ever touch the maptrack tables from the
+   context of the guest which owns them, so if it's uniproc then the
+   lock can't be contended, and is therefore pointless.  Don't bother
+   with that optimisation for now, though, because it's scary and
+   confusing. */
+/* The maptrack lock is top-level: you're not allowed to be holding
+   any other locks when you acquire it. */
+static void
+maptrack_lock(struct grant_table *lgt)
+{
+    spin_lock(&lgt->maptrack_lock);
+}
+
+static void
+maptrack_unlock(struct grant_table *lgt)
+{
+    spin_unlock(&lgt->maptrack_lock);
+}
+
 static inline int
 __get_maptrack_handle(
     struct grant_table *t)
@@ -141,43 +161,30 @@ get_maptrack_handle(
 
     if ( unlikely((handle = __get_maptrack_handle(lgt)) == -1) )
     {
-        spin_lock(&lgt->lock);
+        nr_frames = nr_maptrack_frames(lgt);
+        if ( nr_frames >= max_nr_maptrack_frames() )
+            return -1;
 
-        if ( unlikely((handle = __get_maptrack_handle(lgt)) == -1) )
-        {
-            nr_frames = nr_maptrack_frames(lgt);
-            if ( nr_frames >= max_nr_maptrack_frames() )
-            {
-                spin_unlock(&lgt->lock);
-                return -1;
-            }
+        new_mt = alloc_xenheap_page();
+        if ( new_mt == NULL )
+            return -1;
 
-            new_mt = alloc_xenheap_page();
-            if ( new_mt == NULL )
-            {
-                spin_unlock(&lgt->lock);
-                return -1;
-            }
-
-            clear_page(new_mt);
-
-            new_mt_limit = lgt->maptrack_limit + MAPTRACK_PER_PAGE;
-
-            for ( i = lgt->maptrack_limit; i < new_mt_limit; i++ )
-            {
-                new_mt[i % MAPTRACK_PER_PAGE].ref = i+1;
-                new_mt[i % MAPTRACK_PER_PAGE].flags = 0;
-            }
+        clear_page(new_mt);
 
-            lgt->maptrack[nr_frames] = new_mt;
-            lgt->maptrack_limit      = new_mt_limit;
+        new_mt_limit = lgt->maptrack_limit + MAPTRACK_PER_PAGE;
 
-            gdprintk(XENLOG_INFO,
-                    "Increased maptrack size to %u frames.\n", nr_frames + 1);
-            handle = __get_maptrack_handle(lgt);
+        for ( i = lgt->maptrack_limit; i < new_mt_limit; i++ )
+        {
+            new_mt[i % MAPTRACK_PER_PAGE].ref = i+1;
+            new_mt[i % MAPTRACK_PER_PAGE].flags = 0;
         }
 
-        spin_unlock(&lgt->lock);
+        lgt->maptrack[nr_frames] = new_mt;
+        lgt->maptrack_limit      = new_mt_limit;
+
+        gdprintk(XENLOG_INFO,
+                 "Increased maptrack size to %u frames.\n", nr_frames + 1);
+        handle = __get_maptrack_handle(lgt);
     }
     return handle;
 }
@@ -1508,7 +1515,9 @@ do_grant_table_op(
             guest_handle_cast(uop, gnttab_map_grant_ref_t);
         if ( unlikely(!guest_handle_okay(map, count)) )
             goto out;
+        maptrack_lock(current->domain->grant_table);
         rc = gnttab_map_grant_ref(map, count);
+        maptrack_unlock(current->domain->grant_table);
         break;
     }
     case GNTTABOP_unmap_grant_ref:
@@ -1517,7 +1526,9 @@ do_grant_table_op(
             guest_handle_cast(uop, gnttab_unmap_grant_ref_t);
         if ( unlikely(!guest_handle_okay(unmap, count)) )
             goto out;
+        maptrack_lock(current->domain->grant_table);
         rc = gnttab_unmap_grant_ref(unmap, count);
+        maptrack_unlock(current->domain->grant_table);
         break;
     }
     case GNTTABOP_unmap_and_replace:
@@ -1529,7 +1540,9 @@ do_grant_table_op(
         rc = -ENOSYS;
         if ( unlikely(!replace_grant_supported()) )
             goto out;
+        maptrack_lock(current->domain->grant_table);
         rc = gnttab_unmap_and_replace(unmap, count);
+        maptrack_unlock(current->domain->grant_table);
         break;
     }
     case GNTTABOP_setup_table:
@@ -1600,6 +1613,7 @@ grant_table_create(
     /* Simple stuff. */
     memset(t, 0, sizeof(*t));
     spin_lock_init(&t->lock);
+    spin_lock_init(&t->maptrack_lock);
     t->nr_grant_frames = INITIAL_NR_GRANT_FRAMES;
 
     /* Active grant table. */
@@ -1680,6 +1694,7 @@ gnttab_release_mappings(
 
     for ( handle = 0; handle < gt->maptrack_limit; handle++ )
     {
+        /* Domain is dying, so don't need maptrack lock */
         map = &maptrack_entry(gt, handle);
         if ( !(map->flags & (GNTMAP_device_map|GNTMAP_host_map)) )
             continue;

diff --git a/xen/include/xen/grant_table.h b/xen/include/xen/grant_table.h
index 096af9bb4cad179d8d9b20077acd81cd56d7f3f3..1ab59434ab327ad18bdd2d6aaeaa68d144f07217 100644 (file)
--- a/xen/include/xen/grant_table.h
+++ b/xen/include/xen/grant_table.h
@@ -91,6 +91,8 @@ struct grant_table {
     struct grant_mapping **maptrack;
     unsigned int          maptrack_head;
     unsigned int          maptrack_limit;
+    /* Lock protecting maptrack-related fields. */
+    spinlock_t            maptrack_lock;
     /* Lock protecting updates to active and shared grant tables. */
     spinlock_t            lock;
 };