[PATCH] xen: credit1: fix a race when picking initial pCPU for a vCPU

[Dario Faggioli <dario.faggioli@citrix.com>]

In the Credit1 hunk of 9f358ddd69463 ("xen: Have
schedulers revise initial placement") csched_cpu_pick()
is called without taking the runqueue lock of the
(temporary) pCPU that the vCPU has been assigned to
(e.g., in XEN_DOMCTL_max_vcpus).

However, although 'hidden' in the IS_RUNQ_IDLE() macro,
that function does access the runq (for doing load
balancing calculations), and hence the appropriate lock
must be taken.

Races have been observed, in the form of IS_RUNQ_IDLE()
falling over LIST_POISON.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
---
Cc: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Jan Beulich <JBeulich@suse.com>
---
 xen/common/sched_credit.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 220ff0d..b6f82e8 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -998,9 +998,13 @@ csched_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
 
     BUG_ON( is_idle_vcpu(vc) );
 
-    /* This is safe because vc isn't yet being scheduled */
+    /* csched_cpu_pick() looks in vc->processor's runq, so we need the lock. */
+    lock = vcpu_schedule_lock_irq(vc);
+
     vc->processor = csched_cpu_pick(ops, vc);
 
+    spin_unlock_irq(lock);
+
     lock = vcpu_schedule_lock_irq(vc);
 
     if ( !__vcpu_on_runq(svc) && vcpu_runnable(vc) && !vc->is_running )


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