[xen stable-4.15] xen/sched: fix race in RTDS scheduler

[patchbot@xxxxxxx]

commit 1f679f084fef76810762ee69a584fc1b524be0b6
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Mon Oct 31 13:33:59 2022 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Oct 31 13:33:59 2022 +0100

    xen/sched: fix race in RTDS scheduler
    
    When a domain gets paused the unit runnable state can change to "not
    runnable" without the scheduling lock being involved. This means that
    a specific scheduler isn't involved in this change of runnable state.
    
    In the RTDS scheduler this can result in an inconsistency in case a
    unit is losing its "runnable" capability while the RTDS scheduler's
    scheduling function is active. RTDS will remove the unit from the run
    queue, but doesn't do so for the replenish queue, leading to hitting
    an ASSERT() in replq_insert() later when the domain is unpaused again.
    
    Fix that by removing the unit from the replenish queue as well in this
    case.
    
    Fixes: 7c7b407e7772 ("xen/sched: introduce unit_runnable_state()")
    Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
    Acked-by: Dario Faggioli <dfaggioli@xxxxxxxx>
    master commit: 73c62927f64ecb48f27d06176befdf76b879f340
    master date: 2022-10-21 12:32:23 +0200
---
 xen/common/sched/rt.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/xen/common/sched/rt.c b/xen/common/sched/rt.c
index c24cd2ac32..ec2ca1bebc 100644
--- a/xen/common/sched/rt.c
+++ b/xen/common/sched/rt.c
@@ -1087,6 +1087,7 @@ rt_schedule(const struct scheduler *ops, struct 
sched_unit *currunit,
         else if ( !unit_runnable_state(snext->unit) )
         {
             q_remove(snext);
+            replq_remove(ops, snext);
             snext = rt_unit(sched_idle_unit(sched_cpu));
         }
 
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