xen/557eb620-gnttab-make-the-grant-table-lock-a-read-write-lock.patch

# Commit 40de9fffb4cc0b0485aa3391d72e2220b8e1ce12
# Date 2015-06-15 13:25:20 +0200
# Author David Vrabel <david.vrabel@citrix.com>
# Committer Jan Beulich <jbeulich@suse.com>
gnttab: make the grant table lock a read-write lock

In combination with the per-active entry locks, the grant table lock
can be made a read-write lock since the majority of cases only the
read lock is required. The grant table read lock protects against
changes to the table version or size (which are done with the write
lock held).

The write lock is also required when two active entries must be
acquired.

The double lock is still required when updating IOMMU page tables.

With the lock contention being only on the maptrack lock (unless IOMMU
updates are required), performance and scalability is improved.

Based on a patch originally by Matt Wilson <msw@amazon.com>.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

--- a/docs/misc/grant-tables.txt
+++ b/docs/misc/grant-tables.txt
@@ -83,7 +83,7 @@ is complete.
  ~~~~~~~
  Xen uses several locks to serialize access to the internal grant table state.
 
-  grant_table->lock          : lock used to prevent readers from accessing
+  grant_table->lock          : rwlock used to prevent readers from accessing
                                inconsistent grant table state such as current
                                version, partially initialized active table pages,
                                etc.
@@ -91,34 +91,43 @@ is complete.
   active_grant_entry->lock   : spinlock used to serialize modifications to
                                active entries
 
- The primary lock for the grant table is a spinlock. All functions
- that access members of struct grant_table must acquire the lock
- around critical sections.
+ The primary lock for the grant table is a read/write spinlock. All
+ functions that access members of struct grant_table must acquire a
+ read lock around critical sections. Any modification to the members
+ of struct grant_table (e.g., nr_status_frames, nr_grant_frames,
+ active frames, etc.) must only be made if the write lock is
+ held. These elements are read-mostly, and read critical sections can
+ be large, which makes a rwlock a good choice.
 
  The maptrack free list is protected by its own spinlock. The maptrack
  lock may be locked while holding the grant table lock.
 
  Active entries are obtained by calling active_entry_acquire(gt, ref).
  This function returns a pointer to the active entry after locking its
- spinlock. The caller must hold the grant table lock for the gt in
- question before calling active_entry_acquire(). This is because the
- grant table can be dynamically extended via gnttab_grow_table() while
- a domain is running and must be fully initialized. Once all access to
- the active entry is complete, release the lock by calling
- active_entry_release(act).
+ spinlock. The caller must hold the grant table read lock before
+ calling active_entry_acquire(). This is because the grant table can
+ be dynamically extended via gnttab_grow_table() while a domain is
+ running and must be fully initialized. Once all access to the active
+ entry is complete, release the lock by calling active_entry_release(act).
 
  Summary of rules for locking:
   active_entry_acquire() and active_entry_release() can only be
-  called when holding the relevant grant table's lock. I.e.:
-    spin_lock(&gt->lock);
+  called when holding the relevant grant table's read lock. I.e.:
+    read_lock(&gt->lock);
     act = active_entry_acquire(gt, ref);
     ...
     active_entry_release(act);
-    spin_unlock(&gt->lock);
+    read_unlock(&gt->lock);
 
  Active entries cannot be acquired while holding the maptrack lock.
  Multiple active entries can be acquired while holding the grant table
- lock.
+ _write_ lock.
+
+ Maptrack entries are protected by the corresponding active entry
+ lock.  As an exception, new maptrack entries may be populated without
+ holding the lock, provided the flags field is written last.  This
+ requires any maptrack entry user validates the flags field as
+ non-zero first.
 
 ********************************************************************************
 
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1037,7 +1037,7 @@ int xenmem_add_to_physmap_one(
     switch ( space )
     {
     case XENMAPSPACE_grant_table:
-        spin_lock(&d->grant_table->lock);
+        write_lock(&d->grant_table->lock);
 
         if ( d->grant_table->gt_version == 0 )
             d->grant_table->gt_version = 1;
@@ -1067,7 +1067,7 @@ int xenmem_add_to_physmap_one(
 
         t = p2m_ram_rw;
 
-        spin_unlock(&d->grant_table->lock);
+        write_unlock(&d->grant_table->lock);
         break;
     case XENMAPSPACE_shared_info:
         if ( idx != 0 )
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4587,7 +4587,7 @@ int xenmem_add_to_physmap_one(
                 mfn = virt_to_mfn(d->shared_info);
             break;
         case XENMAPSPACE_grant_table:
-            spin_lock(&d->grant_table->lock);
+            write_lock(&d->grant_table->lock);
 
             if ( d->grant_table->gt_version == 0 )
                 d->grant_table->gt_version = 1;
@@ -4609,7 +4609,7 @@ int xenmem_add_to_physmap_one(
                     mfn = virt_to_mfn(d->grant_table->shared_raw[idx]);
             }
 
-            spin_unlock(&d->grant_table->lock);
+            write_unlock(&d->grant_table->lock);
             break;
         case XENMAPSPACE_gmfn_range:
         case XENMAPSPACE_gmfn:
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -196,7 +196,7 @@ active_entry_acquire(struct grant_table 
 {
     struct active_grant_entry *act;
 
-    ASSERT(spin_is_locked(&t->lock));
+    ASSERT(rw_is_locked(&t->lock));
 
     act = &_active_entry(t, e);
     spin_lock(&act->lock);
@@ -252,25 +252,29 @@ static int __get_paged_frame(unsigned lo
 static inline void
 double_gt_lock(struct grant_table *lgt, struct grant_table *rgt)
 {
+    /*
+     * See mapcount() for why the write lock is also required for the
+     * remote domain.
+     */
     if ( lgt < rgt )
     {
-        spin_lock(&lgt->lock);
-        spin_lock(&rgt->lock);
+        write_lock(&lgt->lock);
+        write_lock(&rgt->lock);
     }
     else
     {
         if ( lgt != rgt )
-            spin_lock(&rgt->lock);
-        spin_lock(&lgt->lock);
+            write_lock(&rgt->lock);
+        write_lock(&lgt->lock);
     }
 }
 
 static inline void
 double_gt_unlock(struct grant_table *lgt, struct grant_table *rgt)
 {
-    spin_unlock(&lgt->lock);
+    write_unlock(&lgt->lock);
     if ( lgt != rgt )
-        spin_unlock(&rgt->lock);
+        write_unlock(&rgt->lock);
 }
 
 static inline int
@@ -528,7 +532,7 @@ static int grant_map_exists(const struct
 {
     unsigned int ref, max_iter;
     
-    ASSERT(spin_is_locked(&rgt->lock));
+    ASSERT(rw_is_locked(&rgt->lock));
 
     max_iter = min(*ref_count + (1 << GNTTABOP_CONTINUATION_ARG_SHIFT),
                    nr_grant_entries(rgt));
@@ -568,15 +572,15 @@ static void mapcount(
     *wrc = *rdc = 0;
 
     /*
-     * Must have the local domain's grant table lock when iterating
-     * over its maptrack entries.
+     * Must have the local domain's grant table write lock when
+     * iterating over its maptrack entries.
      */
-    ASSERT(spin_is_locked(&lgt->lock));
+    ASSERT(rw_is_write_locked(&lgt->lock));
     /*
-     * Must have the remote domain's grant table lock while counting
-     * its active entries.
+     * Must have the remote domain's grant table write lock while
+     * counting its active entries.
      */
-    ASSERT(spin_is_locked(&rd->grant_table->lock));
+    ASSERT(rw_is_write_locked(&rd->grant_table->lock));
 
     for ( handle = 0; handle < lgt->maptrack_limit; handle++ )
     {
@@ -616,6 +620,7 @@ __gnttab_map_grant_ref(
     grant_entry_v2_t *sha2;
     grant_entry_header_t *shah;
     uint16_t *status;
+    bool_t need_iommu;
 
     led = current;
     ld = led->domain;
@@ -661,7 +666,7 @@ __gnttab_map_grant_ref(
     }
 
     rgt = rd->grant_table;
-    spin_lock(&rgt->lock);
+    read_lock(&rgt->lock);
 
     if ( rgt->gt_version == 0 )
         PIN_FAIL(unlock_out, GNTST_general_error,
@@ -735,7 +740,7 @@ __gnttab_map_grant_ref(
     cache_flags = (shah->flags & (GTF_PAT | GTF_PWT | GTF_PCD) );
 
     active_entry_release(act);
-    spin_unlock(&rgt->lock);
+    read_unlock(&rgt->lock);
 
     /* pg may be set, with a refcount included, from __get_paged_frame */
     if ( !pg )
@@ -811,12 +816,14 @@ __gnttab_map_grant_ref(
         goto undo_out;
     }
 
-    double_gt_lock(lgt, rgt);
-
-    if ( gnttab_need_iommu_mapping(ld) )
+    need_iommu = gnttab_need_iommu_mapping(ld);
+    if ( need_iommu )
     {
         unsigned int wrc, rdc;
         int err = 0;
+
+        double_gt_lock(lgt, rgt);
+
         /* We're not translated, so we know that gmfns and mfns are
            the same things, so the IOMMU entry is always 1-to-1. */
         mapcount(lgt, rd, frame, &wrc, &rdc);
@@ -842,12 +849,22 @@ __gnttab_map_grant_ref(
 
     TRACE_1D(TRC_MEM_PAGE_GRANT_MAP, op->dom);
 
+    /*
+     * All maptrack entry users check mt->flags first before using the
+     * other fields so just ensure the flags field is stored last.
+     *
+     * However, if gnttab_need_iommu_mapping() then this would race
+     * with a concurrent mapcount() call (on an unmap, for example)
+     * and a lock is required.
+     */
     mt = &maptrack_entry(lgt, handle);
     mt->domid = op->dom;
     mt->ref   = op->ref;
-    mt->flags = op->flags;
+    wmb();
+    write_atomic(&mt->flags, op->flags);
 
-    double_gt_unlock(lgt, rgt);
+    if ( need_iommu )
+        double_gt_unlock(lgt, rgt);
 
     op->dev_bus_addr = (u64)frame << PAGE_SHIFT;
     op->handle       = handle;
@@ -870,7 +887,7 @@ __gnttab_map_grant_ref(
         put_page(pg);
     }
 
-    spin_lock(&rgt->lock);
+    read_lock(&rgt->lock);
 
     act = active_entry_acquire(rgt, op->ref);
 
@@ -893,7 +910,7 @@ __gnttab_map_grant_ref(
     active_entry_release(act);
 
  unlock_out:
-    spin_unlock(&rgt->lock);
+    read_unlock(&rgt->lock);
     op->status = rc;
     put_maptrack_handle(lgt, handle);
     rcu_unlock_domain(rd);
@@ -943,18 +960,19 @@ __gnttab_unmap_common(
     }
 
     op->map = &maptrack_entry(lgt, op->handle);
-    spin_lock(&lgt->lock);
 
-    if ( unlikely(!op->map->flags) )
+    read_lock(&lgt->lock);
+
+    if ( unlikely(!read_atomic(&op->map->flags)) )
     {
-        spin_unlock(&lgt->lock);
+        read_unlock(&lgt->lock);
         gdprintk(XENLOG_INFO, "Zero flags for handle (%d).\n", op->handle);
         op->status = GNTST_bad_handle;
         return;
     }
 
     dom = op->map->domid;
-    spin_unlock(&lgt->lock);
+    read_unlock(&lgt->lock);
 
     if ( unlikely((rd = rcu_lock_domain_by_id(dom)) == NULL) )
     {
@@ -975,9 +993,10 @@ __gnttab_unmap_common(
     TRACE_1D(TRC_MEM_PAGE_GRANT_UNMAP, dom);
 
     rgt = rd->grant_table;
-    double_gt_lock(lgt, rgt);
 
-    op->flags = op->map->flags;
+    read_lock(&rgt->lock);
+
+    op->flags = read_atomic(&op->map->flags);
     if ( unlikely(!op->flags) || unlikely(op->map->domid != dom) )
     {
         gdprintk(XENLOG_WARNING, "Unstable handle %u\n", op->handle);
@@ -1024,31 +1043,34 @@ __gnttab_unmap_common(
             act->pin -= GNTPIN_hstw_inc;
     }
 
-    if ( gnttab_need_iommu_mapping(ld) )
+ act_release_out:
+    active_entry_release(act);
+ unmap_out:
+    read_unlock(&rgt->lock);
+
+    if ( rc == GNTST_okay && gnttab_need_iommu_mapping(ld) )
     {
         unsigned int wrc, rdc;
         int err = 0;
+
+        double_gt_lock(lgt, rgt);
+
         mapcount(lgt, rd, op->frame, &wrc, &rdc);
         if ( (wrc + rdc) == 0 )
             err = iommu_unmap_page(ld, op->frame);
         else if ( wrc == 0 )
             err = iommu_map_page(ld, op->frame, op->frame, IOMMUF_readable);
+
+        double_gt_unlock(lgt, rgt);
+
         if ( err )
-        {
             rc = GNTST_general_error;
-            goto act_release_out;
-        }
     }
 
     /* If just unmapped a writable mapping, mark as dirtied */
-    if ( !(op->flags & GNTMAP_readonly) )
+    if ( rc == GNTST_okay && !(op->flags & GNTMAP_readonly) )
          gnttab_mark_dirty(rd, op->frame);
 
- act_release_out:
-    active_entry_release(act);
- unmap_out:
-    double_gt_unlock(lgt, rgt);
-
     op->status = rc;
     rcu_unlock_domain(rd);
 }
@@ -1078,8 +1100,8 @@ __gnttab_unmap_common_complete(struct gn
 
     rcu_lock_domain(rd);
     rgt = rd->grant_table;
-    spin_lock(&rgt->lock);
 
+    read_lock(&rgt->lock);
     if ( rgt->gt_version == 0 )
         goto unlock_out;
 
@@ -1145,7 +1167,7 @@ __gnttab_unmap_common_complete(struct gn
  act_release_out:
     active_entry_release(act);
  unlock_out:
-    spin_unlock(&rgt->lock);
+    read_unlock(&rgt->lock);
 
     if ( put_handle )
     {
@@ -1332,11 +1354,13 @@ gnttab_unpopulate_status_frames(struct d
     gt->nr_status_frames = 0;
 }
 
+/*
+ * Grow the grant table. The caller must hold the grant table's
+ * write lock before calling this function.
+ */
 int
 gnttab_grow_table(struct domain *d, unsigned int req_nr_frames)
 {
-    /* d's grant table lock must be held by the caller */
-
     struct grant_table *gt = d->grant_table;
     unsigned int i, j;
 
@@ -1442,7 +1466,7 @@ gnttab_setup_table(
     }
 
     gt = d->grant_table;
-    spin_lock(&gt->lock);
+    write_lock(&gt->lock);
 
     if ( gt->gt_version == 0 )
         gt->gt_version = 1;
@@ -1470,7 +1494,7 @@ gnttab_setup_table(
     }
 
  out3:
-    spin_unlock(&gt->lock);
+    write_unlock(&gt->lock);
  out2:
     rcu_unlock_domain(d);
  out1:
@@ -1512,13 +1536,13 @@ gnttab_query_size(
         goto query_out_unlock;
     }
 
-    spin_lock(&d->grant_table->lock);
+    read_lock(&d->grant_table->lock);
 
     op.nr_frames     = nr_grant_frames(d->grant_table);
     op.max_nr_frames = max_grant_frames;
     op.status        = GNTST_okay;
 
-    spin_unlock(&d->grant_table->lock);
+    read_unlock(&d->grant_table->lock);
 
  
  query_out_unlock:
@@ -1544,7 +1568,7 @@ gnttab_prepare_for_transfer(
     union grant_combo   scombo, prev_scombo, new_scombo;
     int                 retries = 0;
 
-    spin_lock(&rgt->lock);
+    read_lock(&rgt->lock);
 
     if ( rgt->gt_version == 0 )
     {
@@ -1595,11 +1619,11 @@ gnttab_prepare_for_transfer(
         scombo = prev_scombo;
     }
 
-    spin_unlock(&rgt->lock);
+    read_unlock(&rgt->lock);
     return 1;
 
  fail:
-    spin_unlock(&rgt->lock);
+    read_unlock(&rgt->lock);
     return 0;
 }
 
@@ -1614,6 +1638,7 @@ gnttab_transfer(
     struct gnttab_transfer gop;
     unsigned long mfn;
     unsigned int max_bitsize;
+    struct active_grant_entry *act;
 
     for ( i = 0; i < count; i++ )
     {
@@ -1791,7 +1816,8 @@ gnttab_transfer(
         TRACE_1D(TRC_MEM_PAGE_GRANT_TRANSFER, e->domain_id);
 
         /* Tell the guest about its new page frame. */
-        spin_lock(&e->grant_table->lock);
+        read_lock(&e->grant_table->lock);
+        act = active_entry_acquire(e->grant_table, gop.ref);
 
         if ( e->grant_table->gt_version == 1 )
         {
@@ -1809,7 +1835,8 @@ gnttab_transfer(
         shared_entry_header(e->grant_table, gop.ref)->flags |=
             GTF_transfer_completed;
 
-        spin_unlock(&e->grant_table->lock);
+        active_entry_release(act);
+        read_unlock(&e->grant_table->lock);
 
         rcu_unlock_domain(e);
 
@@ -1847,7 +1874,7 @@ __release_grant_for_copy(
     released_read = 0;
     released_write = 0;
 
-    spin_lock(&rgt->lock);
+    read_lock(&rgt->lock);
 
     act = active_entry_acquire(rgt, gref);
     sha = shared_entry_header(rgt, gref);
@@ -1889,7 +1916,7 @@ __release_grant_for_copy(
     }
 
     active_entry_release(act);
-    spin_unlock(&rgt->lock);
+    read_unlock(&rgt->lock);
 
     if ( td != rd )
     {
@@ -1947,7 +1974,7 @@ __acquire_grant_for_copy(
 
     *page = NULL;
 
-    spin_lock(&rgt->lock);
+    read_lock(&rgt->lock);
 
     if ( rgt->gt_version == 0 )
         PIN_FAIL(gt_unlock_out, GNTST_general_error,
@@ -2023,20 +2050,20 @@ __acquire_grant_for_copy(
              * here and reacquire
              */
             active_entry_release(act);
-            spin_unlock(&rgt->lock);
+            read_unlock(&rgt->lock);
 
             rc = __acquire_grant_for_copy(td, trans_gref, rd->domain_id,
                                           readonly, &grant_frame, page,
                                           &trans_page_off, &trans_length, 0);
 
-            spin_lock(&rgt->lock);
+            read_lock(&rgt->lock);
             act = active_entry_acquire(rgt, gref);
 
             if ( rc != GNTST_okay ) {
                 __fixup_status_for_copy_pin(act, status);
                 rcu_unlock_domain(td);
                 active_entry_release(act);
-                spin_unlock(&rgt->lock);
+                read_unlock(&rgt->lock);
                 return rc;
             }
 
@@ -2049,7 +2076,7 @@ __acquire_grant_for_copy(
                 __fixup_status_for_copy_pin(act, status);
                 rcu_unlock_domain(td);
                 active_entry_release(act);
-                spin_unlock(&rgt->lock);
+                read_unlock(&rgt->lock);
                 put_page(*page);
                 return __acquire_grant_for_copy(rd, gref, ldom, readonly,
                                                 frame, page, page_off, length,
@@ -2118,7 +2145,7 @@ __acquire_grant_for_copy(
     *frame = act->frame;
 
     active_entry_release(act);
-    spin_unlock(&rgt->lock);
+    read_unlock(&rgt->lock);
     return rc;
  
  unlock_out_clear:
@@ -2133,7 +2160,7 @@ __acquire_grant_for_copy(
     active_entry_release(act);
 
  gt_unlock_out:
-    spin_unlock(&rgt->lock);
+    read_unlock(&rgt->lock);
 
     return rc;
 }
@@ -2307,7 +2334,7 @@ gnttab_set_version(XEN_GUEST_HANDLE_PARA
     if ( gt->gt_version == op.version )
         goto out;
 
-    spin_lock(&gt->lock);
+    write_lock(&gt->lock);
     /* Make sure that the grant table isn't currently in use when we
        change the version number, except for the first 8 entries which
        are allowed to be in use (xenstore/xenconsole keeps them mapped).
@@ -2392,7 +2419,7 @@ gnttab_set_version(XEN_GUEST_HANDLE_PARA
     gt->gt_version = op.version;
 
 out_unlock:
-    spin_unlock(&gt->lock);
+    write_unlock(&gt->lock);
 
 out:
     op.version = gt->gt_version;
@@ -2448,7 +2475,7 @@ gnttab_get_status_frames(XEN_GUEST_HANDL
 
     op.status = GNTST_okay;
 
-    spin_lock(&gt->lock);
+    read_lock(&gt->lock);
 
     for ( i = 0; i < op.nr_frames; i++ )
     {
@@ -2457,7 +2484,7 @@ gnttab_get_status_frames(XEN_GUEST_HANDL
             op.status = GNTST_bad_virt_addr;
     }
 
-    spin_unlock(&gt->lock);
+    read_unlock(&gt->lock);
 out2:
     rcu_unlock_domain(d);
 out1:
@@ -2507,7 +2534,7 @@ __gnttab_swap_grant_ref(grant_ref_t ref_
     struct active_grant_entry *act_b = NULL;
     s16 rc = GNTST_okay;
 
-    spin_lock(&gt->lock);
+    write_lock(&gt->lock);
 
     if ( gt->gt_version == 0 )
         PIN_FAIL(out, GNTST_general_error, "grant table not yet set up\n");
@@ -2558,7 +2585,7 @@ out:
         active_entry_release(act_b);
     if ( act_a != NULL )
         active_entry_release(act_a);
-    spin_unlock(&gt->lock);
+    write_unlock(&gt->lock);
 
     rcu_unlock_domain(d);
 
@@ -2629,12 +2656,12 @@ static int __gnttab_cache_flush(gnttab_c
 
     if ( d != owner )
     {
-        spin_lock(&owner->grant_table->lock);
+        read_lock(&owner->grant_table->lock);
 
         ret = grant_map_exists(d, owner->grant_table, mfn, ref_count);
         if ( ret != 0 )
         {
-            spin_unlock(&owner->grant_table->lock);
+            read_unlock(&owner->grant_table->lock);
             rcu_unlock_domain(d);
             put_page(page);
             return ret;
@@ -2654,7 +2681,7 @@ static int __gnttab_cache_flush(gnttab_c
         ret = 0;
 
     if ( d != owner )
-        spin_unlock(&owner->grant_table->lock);
+        read_unlock(&owner->grant_table->lock);
     unmap_domain_page(v);
     put_page(page);
 
@@ -2873,7 +2900,7 @@ grant_table_create(
         goto no_mem_0;
 
     /* Simple stuff. */
-    spin_lock_init(&t->lock);
+    rwlock_init(&t->lock);
     spin_lock_init(&t->maptrack_lock);
     t->nr_grant_frames = INITIAL_NR_GRANT_FRAMES;
 
@@ -2983,7 +3010,7 @@ gnttab_release_mappings(
         }
 
         rgt = rd->grant_table;
-        spin_lock(&rgt->lock);
+        read_lock(&rgt->lock);
 
         act = active_entry_acquire(rgt, ref);
         sha = shared_entry_header(rgt, ref);
@@ -3044,7 +3071,7 @@ gnttab_release_mappings(
             gnttab_clear_flag(_GTF_reading, status);
 
         active_entry_release(act);
-        spin_unlock(&rgt->lock);
+        read_unlock(&rgt->lock);
 
         rcu_unlock_domain(rd);
 
@@ -3092,7 +3119,7 @@ static void gnttab_usage_print(struct do
     printk("      -------- active --------       -------- shared --------\n");
     printk("[ref] localdom mfn      pin          localdom gmfn     flags\n");
 
-    spin_lock(&gt->lock);
+    read_lock(&gt->lock);
 
     if ( gt->gt_version == 0 )
         goto out;
@@ -3145,7 +3172,7 @@ static void gnttab_usage_print(struct do
     }
 
  out:
-    spin_unlock(&gt->lock);
+    read_unlock(&gt->lock);
 
     if ( first )
         printk("grant-table for remote domain:%5d ... "
--- a/xen/include/xen/grant_table.h
+++ b/xen/include/xen/grant_table.h
@@ -64,6 +64,11 @@ struct grant_mapping {
 
 /* Per-domain grant information. */
 struct grant_table {
+    /*
+     * Lock protecting updates to grant table state (version, active
+     * entry list, etc.)
+     */
+    rwlock_t              lock;
     /* Table size. Number of frames shared with guest */
     unsigned int          nr_grant_frames;
     /* Shared grant table (see include/public/grant_table.h). */
@@ -84,8 +89,6 @@ struct grant_table {
     unsigned int          maptrack_limit;
     /* Lock protecting the maptrack page list, head, and limit */
     spinlock_t            maptrack_lock;
-    /* Lock protecting updates to active and shared grant tables. */
-    spinlock_t            lock;
     /* The defined versions are 1 and 2.  Set to 0 if we don't know
        what version to use yet. */
     unsigned              gt_version;
@@ -103,7 +106,7 @@ gnttab_release_mappings(
     struct domain *d);
 
 /* Increase the size of a domain's grant table.
- * Caller must hold d's grant table lock.
+ * Caller must hold d's grant table write lock.
  */
 int
 gnttab_grow_table(struct domain *d, unsigned int req_nr_frames);