[xen stable-4.18] xen/sched: fix sched_move_domain()

[patchbot@xxxxxxx]

commit 3f9390fea5c51a6d64596d295902d28931eeca4c
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Thu Nov 23 12:13:53 2023 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Nov 23 12:13:53 2023 +0100

    xen/sched: fix sched_move_domain()
    
    When moving a domain out of a cpupool running with the credit2
    scheduler and having multiple run-queues, the following ASSERT() can
    be observed:
    
    (XEN) Xen call trace:
    (XEN)    [<ffff82d04023a700>] R credit2.c#csched2_unit_remove+0xe3/0xe7
    (XEN)    [<ffff82d040246adb>] S sched_move_domain+0x2f3/0x5b1
    (XEN)    [<ffff82d040234cf7>] S 
cpupool.c#cpupool_move_domain_locked+0x1d/0x3b
    (XEN)    [<ffff82d040236025>] S cpupool_move_domain+0x24/0x35
    (XEN)    [<ffff82d040206513>] S domain_kill+0xa5/0x116
    (XEN)    [<ffff82d040232b12>] S do_domctl+0xe5f/0x1951
    (XEN)    [<ffff82d0402276ba>] S timer.c#timer_lock+0x69/0x143
    (XEN)    [<ffff82d0402dc71b>] S pv_hypercall+0x44e/0x4a9
    (XEN)    [<ffff82d0402012b7>] S lstar_enter+0x137/0x140
    (XEN)
    (XEN)
    (XEN) ****************************************
    (XEN) Panic on CPU 1:
    (XEN) Assertion 'svc->rqd == c2rqd(sched_unit_master(unit))' failed at 
common/sched/credit2.c:1159
    (XEN) ****************************************
    
    This is happening as sched_move_domain() is setting a different cpu
    for a scheduling unit without telling the scheduler. When this unit is
    removed from the scheduler, the ASSERT() will trigger.
    
    In non-debug builds the result is usually a clobbered pointer, leading
    to another crash a short time later.
    
    Fix that by swapping the two involved actions (setting another cpu and
    removing the unit from the scheduler).
    
    Link: https://github.com/Dasharo/dasharo-issues/issues/488
    Fixes: 70fadc41635b ("xen/cpupool: support moving domain between cpupools 
with different granularity")
    Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxx>
    master commit: 4709ec82917668c2df958ef91b4f21c049c76bee
    master date: 2023-11-20 10:49:29 +0100
---
 xen/common/sched/core.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
index 12deefa745..eba0cea4bb 100644
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -732,18 +732,20 @@ int sched_move_domain(struct domain *d, struct cpupool *c)
     old_domdata = d->sched_priv;
 
     /*
-     * Temporarily move all units to same processor to make locking
-     * easier when moving the new units to the new processors.
+     * Remove all units from the old scheduler, and temporarily move them to
+     * the same processor to make locking easier when moving the new units to
+     * new processors.
      */
     new_p = cpumask_first(d->cpupool->cpu_valid);
     for_each_sched_unit ( d, unit )
     {
-        spinlock_t *lock = unit_schedule_lock_irq(unit);
+        spinlock_t *lock;
+
+        sched_remove_unit(old_ops, unit);
 
+        lock = unit_schedule_lock_irq(unit);
         sched_set_res(unit, get_sched_res(new_p));
         spin_unlock_irq(lock);
-
-        sched_remove_unit(old_ops, unit);
     }
 
     old_units = d->sched_unit_list;
--
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