[xen stable-4.20] sched/null: avoid another crash after failed domU creation

[patchbot@xxxxxxx]

commit 5b5a11c6c846e9908c9dcce12ff436106e3c5b87
Author:     Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
AuthorDate: Tue Apr 29 11:40:06 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Apr 29 11:40:06 2025 +0200

    sched/null: avoid another crash after failed domU creation
    
    The following sequence of events may lead a debug build of Xen to crash
    when using the null scheduler:
    
    1. domain creation (e.g. d1) failed due to bad configuration
    2. complete_domain_destroy() was deferred
    3. domain creation (e.g. d2) succeeds
    
    At this point, d2 is running, while the zombie d1 is not fully cleaned
    up:
    
    (XEN) Online Cpus: 0-3
    (XEN) Cpupool 0:
    (XEN) Cpus: 0-3
    (XEN) Scheduling granularity: cpu, 1 CPU per sched-resource
    (XEN) Scheduler: null Scheduler (null)
    (XEN)   cpus_free = 3
    (XEN) Domain info:
    (XEN)   Domain: 0
    (XEN)     1: [0.0] pcpu=0
    (XEN)     2: [0.1] pcpu=1
    (XEN)   Domain: 1
    (XEN)     3: [1.0] pcpu=2
    (XEN)   Domain: 2
    (XEN)     4: [2.0] pcpu=2
    
    4. complete_domain_destroy() gets called for d1 and triggers the
    following:
    
    (XEN) Xen call trace:
    (XEN)    [<00000a0000322ed4>] null.c#unit_deassign+0x2d8/0xb70 (PC)
    (XEN)    [<00000a000032457c>] null.c#null_unit_remove+0x670/0xba8 (LR)
    (XEN)    [<00000a000032457c>] null.c#null_unit_remove+0x670/0xba8
    (XEN)    [<00000a0000336404>] sched_destroy_vcpu+0x354/0x8fc
    (XEN)    [<00000a0000227324>] domain.c#complete_domain_destroy+0x11c/0x49c
    (XEN)    [<00000a000029fbd0>] rcupdate.c#rcu_do_batch+0x94/0x3d0
    (XEN)    [<00000a00002a10c0>] rcupdate.c#__rcu_process_callbacks+0x160/0x5f4
    (XEN)    [<00000a00002a1e60>] rcupdate.c#rcu_process_callbacks+0xcc/0x1b0
    (XEN)    [<00000a00002a3460>] softirq.c#__do_softirq+0x1f4/0x3d8
    (XEN)    [<00000a00002a37c4>] do_softirq+0x14/0x1c
    (XEN)    [<00000a0000465260>] traps.c#check_for_pcpu_work+0x30/0xb8
    (XEN)    [<00000a000046bb08>] leave_hypervisor_to_guest+0x28/0x198
    (XEN)    [<00000a0000409c84>] entry.o#guest_sync_slowpath+0xac/0xd8
    (XEN)
    (XEN) ****************************************
    (XEN) Panic on CPU 0:
    (XEN) Assertion 'npc->unit == unit' failed at common/sched/null.c:383
    (XEN) ****************************************
    
    Fix by skipping unit_deassign() when the unit to be removed does not
    match the pcpu's currently assigned unit.
    
    Fixes: c2eae2614c8f ("sched/null: avoid crash after failed domU creation")
    Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
    master commit: 54fe207f29f86c4226a62a4dd289f10d9d2abc40
    master date: 2025-04-07 12:17:31 +0200
---
 xen/common/sched/null.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/common/sched/null.c b/xen/common/sched/null.c
index 7e31440e5b..c8e327e3cd 100644
--- a/xen/common/sched/null.c
+++ b/xen/common/sched/null.c
@@ -557,7 +557,7 @@ static void cf_check null_unit_remove(
 
     cpu = sched_unit_master(unit);
     npc = get_sched_res(cpu)->sched_priv;
-    if ( npc->unit )
+    if ( npc->unit == unit )
         unit_deassign(prv, unit);
 
  out:
--
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