[Xen-changelog] [xen-3.4-testing] Fix up the synchronisation around grant table map track handles.

["Xen patchbot-3.4-testing" <patchbot-3.4-testing@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1243863787 -3600
# Node ID 28007c168f077aef994a541b08aedf2fad0368d4
# Parent  23c0bd0819d6fc13b6d1c107a008c034574d0d5e
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@xxxxxxxxxx>
xen-unstable changeset:   19658:28a197617286
xen-unstable date:        Wed May 27 11:29:38 2009 +0100
---
 xen/common/grant_table.c      |   89 ++++++++++++++++++++++++------------------
 xen/include/xen/grant_table.h |    2 
 2 files changed, 54 insertions(+), 37 deletions(-)

diff -r 23c0bd0819d6 -r 28007c168f07 xen/common/grant_table.c
--- a/xen/common/grant_table.c  Mon Jun 01 14:42:36 2009 +0100
+++ b/xen/common/grant_table.c  Mon Jun 01 14:43:07 2009 +0100
@@ -111,6 +111,26 @@ static unsigned inline int max_nr_maptra
 #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);
-
-        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 )
-            {
-                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;
-            }
-
-            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);
-        }
-
-        spin_unlock(&lgt->lock);
+        nr_frames = nr_maptrack_frames(lgt);
+        if ( nr_frames >= max_nr_maptrack_frames() )
+            return -1;
+
+        new_mt = alloc_xenheap_page();
+        if ( new_mt == NULL )
+            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;
+        }
+
+        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 -r 23c0bd0819d6 -r 28007c168f07 xen/include/xen/grant_table.h
--- a/xen/include/xen/grant_table.h     Mon Jun 01 14:42:36 2009 +0100
+++ b/xen/include/xen/grant_table.h     Mon Jun 01 14:43:07 2009 +0100
@@ -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;
 };

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