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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan