Re: [PATCH] sched/null: avoid another crash after failed domU creation

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.04.2025 09:38, Jürgen Groß wrote:
> On 04.04.25 09:24, Jan Beulich wrote:
>> On 03.04.2025 23:48, Stewart Hildebrand wrote:
>>> 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.
>>>
>>> Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
>>
>> Just one remark here: This almost certainly wants a Fixes: tag.
> 
> This will probably be d671da3be3ca (the initial NULL scheduler commit).

The code in question was added by c2eae2614c8f ("sched/null: avoid crash
after failed domU creation").

Jan