[Xen-API] [PATCH] Fix race in memory-offset calibration code

[David Scott <dave.scott@xxxxxxxxxxxxx>]

# HG changeset patch
# User David Scott <dave.scott@xxxxxxxxxxxxx>
# Date 1259942648 0
# Node ID 391cde93edca48ad1f651a944462d15b8a74f2d3
# Parent  4da34ecf1254e82d1e783b81395aaf4e4b2a69e6
Fix a potential race in the memory-offset calibration code: resample 
total_pages after noticing feature-balloon.

The race was easily visible in the hypercall simulator but probably less common 
in real life. The sequence was:
1. domain is using < startmem
2. domain_getinfolist
3. domain is now using startmem
4. domain writes feature-balloon
5. memory_offset <- old total_pages from (2) - target

Now we call domain_getinfo between (4) and (5).

Signed-off-by: David Scott <dave.scott@xxxxxxxxxxxxx>

diff -r 4da34ecf1254 -r 391cde93edca ocaml/xenops/squeeze_xen.ml
--- a/ocaml/xenops/squeeze_xen.ml       Fri Dec 04 16:04:07 2009 +0000
+++ b/ocaml/xenops/squeeze_xen.ml       Fri Dec 04 16:04:08 2009 +0000
@@ -276,11 +276,15 @@
                                            try
                                              Domain.get_memory_offset cnx 
di.Xc.domid
                                            with Xb.Noent ->
-                                             let target_kib = 
Domain.get_target cnx di.Xc.domid in
-                                             let offset_kib = 
memory_actual_kib -* target_kib in
-                                             debug "domid %d just exposed 
feature-balloon; calibrating memory-offset = %Ld KiB" di.Xc.domid offset_kib;
-                                             Domain.set_memory_offset_noexn 
cnx di.Xc.domid offset_kib;
-                                             offset_kib
+                                                       (* Our 
memory_actual_kib value was sampled before reading xenstore which means there 
is a slight race.
+                                                          The race is probably 
only noticable in the hypercall simulator. However we can fix it by resampling
+                                                          memory_actual 
*after* noticing the feature-balloon flag. *)
+                                                       let target_kib = 
Domain.get_target cnx di.Xc.domid in
+                                                       let memory_actual_kib' 
= Xc.pages_to_kib (Int64.of_nativeint (Xc.domain_getinfo xc 
di.Xc.domid).Xc.total_memory_pages) in
+                                                       let offset_kib = 
memory_actual_kib' -* target_kib in
+                                                       debug "domid %d just 
exposed feature-balloon; calibrating memory-offset = %Ld KiB" di.Xc.domid 
offset_kib;
+                                                       
Domain.set_memory_offset_noexn cnx di.Xc.domid offset_kib;
+                                                       offset_kib
                                          end in
                                        let memory_actual_kib = 
memory_actual_kib -* offset_kib in
 
1 file changed, 9 insertions(+), 5 deletions(-)
ocaml/xenops/squeeze_xen.ml |   14 +++++++++-----

Attachment: xen-api.hg.patch
Description: Text Data

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