Re: [Xen-devel] [PATCHv9 3/4] gnttab: make the grant table lock a read-write lock

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 21/05/15 14:36, David Vrabel wrote:
> On 21/05/15 11:32, Jan Beulich wrote:
>>>>> On 20.05.15 at 17:54, <david.vrabel@xxxxxxxxxx> wrote:
>>> @@ -842,8 +845,6 @@ __gnttab_map_grant_ref(
>>>      mt->ref   = op->ref;
>>>      mt->flags = op->flags;
>>>  
>>> -    double_gt_unlock(lgt, rgt);
>>
>> Don't the mt-> updates above need some kind of protection?
> 
> It depends:
> 
> If not gnttab_need_iommu_mapping() but we only need a write barrier
> before the mt->flags store.
> 
> If gnttab_need_iommu_mapping() then a lock is required to prevent racing
> with concurrent mapcount() calls.  This is not a path I care about so
> its easiest to just keep the double lock around this.

Like so:

@@ -806,12 +806,13 @@ __gnttab_map_grant_ref(
         goto undo_out;
     }

-    double_gt_lock(lgt, rgt);
-
     if ( gnttab_need_iommu_mapping(ld) )
     {
         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);
@@ -837,12 +838,22 @@ __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 ( gnttab_need_iommu_mapping(ld) )
+        double_gt_unlock(lgt, rgt);

     op->dev_bus_addr = (u64)frame << PAGE_SHIFT;
     op->handle       = handle;

David

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