[PATCH 1/2] xen/credit: fix race with schedule_cpu_rm() in csched_timer()

[Juergen Gross <jgross@xxxxxxxx>]

When removing a CPU from a cpupool running the credit scheduler, a
race might happen between schedule_cpu_rm() and csched_timer(),
resulting in a NULL dereference.

The timer associated with csched_timer() is killed only in
csched_deinit_pdata(), which is called by schedule_cpu_rm() after
setting the scheduler's per-cpu data to NULL AND after enabling
interrupts again. This can result in the timer to fire before it is
being killed, so csched_timer() needs to test the per-cpu data to be
set before accessing it.

Fixes: 78be3dbbfefa ("cpupools [1/6]: hypervisor changes")
Reported-by: Dietmar Hahn <dietmar.hahn@xxxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 xen/common/sched/credit.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/xen/common/sched/credit.c b/xen/common/sched/credit.c
index 328c802d0c..4dde2ede12 100644
--- a/xen/common/sched/credit.c
+++ b/xen/common/sched/credit.c
@@ -1589,6 +1589,10 @@ static void cf_check csched_tick(void *_cpu)
     struct csched_pcpu *spc = CSCHED_PCPU(cpu);
     struct csched_private *prv = CSCHED_PRIV(sr->scheduler);
 
+    /* Handle race of timer disabling vs. firing when switching scheduler. */
+    if ( !spc )
+        return;
+
     spc->tick++;
 
     /*
-- 
2.55.0