Re: domU suspend issue - freeze processes failed - Linux 6.16

[Jürgen Groß <jgross@xxxxxxxx>]

On 22.08.25 16:42, Marek Marczykowski-Górecki wrote:
> On Fri, Aug 22, 2025 at 04:39:33PM +0200, Marek Marczykowski-Górecki wrote:
> > Hi,
> >
> > When suspending domU I get the following issue:
> >
> >      Freezing user space processes
> >      Freezing user space processes failed after 20.004 seconds (1 tasks 
> > refusing to freeze, wq_busy=0):
> >      task:xl              state:D stack:0     pid:466   tgid:466   ppid:1      
> > task_flags:0x400040 flags:0x00004006
> >      Call Trace:
> >       <TASK>
> >       __schedule+0x2f3/0x780
> >       schedule+0x27/0x80
> >       schedule_preempt_disabled+0x15/0x30
> >       __mutex_lock.constprop.0+0x49f/0x880
> >       unregister_xenbus_watch+0x216/0x230
> >       xenbus_write_watch+0xb9/0x220
> >       xenbus_file_write+0x131/0x1b0
> >       vfs_writev+0x26c/0x3d0
> >       ? do_writev+0xeb/0x110
> >       do_writev+0xeb/0x110
> >       do_syscall_64+0x84/0x2c0
> >       ? do_syscall_64+0x200/0x2c0
> >       ? generic_handle_irq+0x3f/0x60
> >       ? syscall_exit_work+0x108/0x140
> >       ? do_syscall_64+0x200/0x2c0
> >       ? __irq_exit_rcu+0x4c/0xe0
> >       entry_SYSCALL_64_after_hwframe+0x76/0x7e
> >      RIP: 0033:0x79b618138642
> >      RSP: 002b:00007fff9a192fc8 EFLAGS: 00000246 ORIG_RAX: 0000000000000014
> >      RAX: ffffffffffffffda RBX: 00000000024fd490 RCX: 000079b618138642
> >      RDX: 0000000000000003 RSI: 00007fff9a193120 RDI: 0000000000000014
> >      RBP: 00007fff9a193000 R08: 0000000000000000 R09: 0000000000000000
> >      R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000014
> >      R13: 00007fff9a193120 R14: 0000000000000003 R15: 0000000000000000
> >       </TASK>
> >      OOM killer enabled.
> >      Restarting tasks: Starting
> >      Restarting tasks: Done
> >      xen:manage: do_suspend: freeze processes failed -16
> >
> > The process in question is `xl devd` daemon. It's a domU serving a
> > xenvif backend.
> >
> > I noticed it on 6.16.1, but looking at earlier test logs I see it with
> > 6.16-rc6 already (but interestingly, not 6.16-rc2 yet? feels weird given
> > seemingly no relevant changes between rc2 and rc6).
> I forgot to include link for (a little) more details:
> https://github.com/QubesOS/qubes-linux-kernel/pull/1157
>
> Especially, there is another call trace with panic_on_warn enabled -
> slightly different, but looks related.
I'm pretty sure the PV variant for suspending is just wrong: it is calling
dpm_suspend_start() from do_suspend() without taking the required
system_transition_mutex, resulting in the WARN() in pm_restrict_gfp_mask().

It might be as easy as just adding the mutex() call to do_suspend(), but I'm
really not sure that will be a proper fix.


Juergen

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