[Xen-devel] [PATCH 06/19] xen: credit2: read NOW() with the proper runq lock held

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

Yet another situation very similar to 779511f4bf5ae
("sched: avoid races on time values read from NOW()").

In fact, when more than one runqueue is involved, we need
to make sure that the following does not happen:
 1. take the lock of 1st runq
 2. now = NOW()
 3. take the lock of 2nd runq
 4. use now

as, if we have to wait at step 3, the value in now may
be stale when we get to use it at step 4.

While there, fix the style of a label.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 xen/common/sched_credit2.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 9e8e561..50f8dfd 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -1361,7 +1361,7 @@ static void balance_load(const struct scheduler *ops, int 
cpu, s_time_t now)
 
     __update_runq_load(ops, st.lrqd, 0, now);
 
-retry:
+ retry:
     if ( !spin_trylock(&prv->lock) )
         return;
 
@@ -1377,7 +1377,8 @@ retry:
              || !spin_trylock(&st.orqd->lock) )
             continue;
 
-        __update_runq_load(ops, st.orqd, 0, now);
+        /* Use a value of NOW() sampled after taking orqd's lock. */
+        __update_runq_load(ops, st.orqd, 0, NOW());
     
         delta = st.lrqd->b_avgload - st.orqd->b_avgload;
         if ( delta < 0 )
@@ -1435,6 +1436,8 @@ retry:
     if ( unlikely(st.orqd->id < 0) )
         goto out_up;
 
+    now = NOW();
+
     /* Look for "swap" which gives the best load average
      * FIXME: O(n^2)! */
 


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