Re: [RFC PATCH] exit: Skip panic in do_exit() during poweroff

["Tze-nan Wu (吳澤南)" <Tze-nan.Wu@mediatek.com>]

On Thu, 2025-04-10 at 23:05 +0200, Oleg Nesterov wrote:
> 
> External email : Please do not click links or open attachments until
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> 
> 
> Well...
> 
> Let me repeat. I don't understand the kernel/reboot.c paths, you can
> safely ignore me.
> 
> But I still think that you target the wrong goal. Quite possibly I am
> wrong.
> 
> On 04/10, Tze-nan Wu wrote:
> > 
> > If PID 1 exits due to the unreliable userspace after
> > kernel_power_off()
> > invoked,
> 
> Why. Why the global init does do_exit()? It should not, that is all.
> It doesn't matter if it is single threaded or not.
> 
> As for sys_reboot(), I think that kernel_power_off() must be
> __noreturn,
> and sys_reboot() should use BUG() after
> LINUX_REBOOT_CMD_POWER_OFF/_HALT
> instead of do_exit().
> 

Yes, kernel_power_off() should not return, but this is the case only if
kernel_power_off() is invoked by PID 1 through sys_reboot().
If kernel_power_off() is invoked by a kernel thread (e.g., the thermal
kernel module) other than PID 1, then do_exit() could possibly be
invoked by PID 1 after kernel_power_off() on another CPU. (shown as
below)

cpu 1 (thermal ko)                cpu 2 (PID 1)
-----------------                 ---------------
kernel_power_off                      ...
->ufshcd_wl_shutdown(UFS down)       ...
...                                PID 1 page fault
...                                fail to handle page fault (UFS down)
...                                send SIGBUS to PID 1
...                                PID 1 trap to do_exit()
...                                panic()
->machine_power_off()
  -> smp_send_stop() //stop other CPUs

We have encounter this scenario several times in a low rate on kernel-
6.12.

> If nothing else. do_exit() also does debug_check_no_locks_held() and
> sys_reboot() calls do_exit() with system_transition_mutex held.
> 
> IOW. IMO, it is not that do_exit() needs some changes. The very fact
> that the global init does do_exit() is wrong, this should be fixed.
> 
I'm not an expert on UFS, but if we want to prevent entering do_exit()
after kernel_power_off(), perhaps moving ufshcd_wl_shutdown() after 
smp_send_stop() could help.
Since the userspace process running on the other CPUs before
smp_send_stop() could still access the UFS.
But not sure if that's possible...

Tze-nan
> But again, again, I can't really comment.
> 
> Oleg.
> 
> > the panic follow by the last thread of global init exited in
> > do_exit() will stop the kernel_power_off() procedure, turn a
> > shutdown
> > behavior into panic flow(reboot).
> > 
> > Add a condition check to ensure that the panic triggered by the
> > last
> > thread of the global init exiting, only occurs while:
> > ( system_state != SYSTEM_POWER_OFF and system_state !=
> > SYSTEM_RESTART).
> > Otherwise, WARN() instead.
> > 
> > [On Android 16 with arm64 arch]
> > Here's a scenario where the global init exits during
> > kernel_power_off:
> > If PID 1 encounters a page fault after kernel_power_off() has been
> > invoked, the kernel will fail to handle the page fault because the
> > disk(UFS) has already shut down.
> > Consequently, the kernel will send a SIGBUS to PID 1 to indicate
> > the
> > page fault failure, and ultimately, the panic will occur after PID
> > 1
> > exits due to receiving the SIGBUS.
> > 
> >             cpu1                           cpu2
> >           ----------                     ----------
> >     kernel_power_off() start
> >         UFS shutdown
> >             ...                              PID 1 page fault
> >             ...                    page fault handle failure
> >             ...                                    PID 1 received
> > SIGBUS
> >             ...                             panic
> >    kernel_power_off() not done
> > 
> > Backtrace while PID 1 received signal 7:
> >    init-1 [007] d..1 41239.922385: \
> >       signal_generate: sig=7 errno=0 code=2 comm=init pid=1 grp=0
> > res=0
> >    init-1 [007] d..1 41239.922389: kernel_stack: <stack trace>
> >    => __send_signal_locked
> >    => send_signal_locked
> >    => force_sig_info_to_task
> >    => force_sig_fault
> >    => arm64_force_sig_fault
> >    => do_page_fault
> >    => do_translation_fault
> >    => do_mem_abort
> >    => el0_ia
> >    => el0t_64_sync_handler
> > 
> > Simplified kernel log:
> > kernel_power_off() invoked by pt_notify_thread.
> > [41239.526109] pt_notify_threa: reboot set flag, old value
> > 0x********,
> > *.
> > [41239.526114] pt_notify_threa: reboot set flag new value
> > 0x********.
> > UFS reject I/O after kerenl_power_off.
> > [41239.686411]  scsi +scsi******** apexd: sd* ******** rejecting
> > I/O to
> > offline device.
> > Lots of I/O error & erofs error happened after kernel_power_off().
> > [41239.690312] apexd: I/O error, dev sdc, sector ******* op
> > ***:(READ)
> > flags 0x**** phys_seg ** prio class 0.
> > [41239.690465] apexd: I/O error, dev sdc, sector ******* op
> > ***:(READ)
> > flags 0x**** phys_seg ** prio class 0.
> > ...
> > ...
> > [41239.922265] init: erofs: (device ****): z_erofs_read_folio: read
> > error * @ *** of nid ********.
> > [41239.922341] init: erofs: (device ****): z_erofs_read_folio: read
> > error * @ *** of nid ********.
> > Finally device panic due to PID 1 received SIGBUS.
> > [41239.923789] init: Kernel panic - not syncing: Attempted to kill
> > init!
> > exitcode=0x00000007
> > 
> > Fixes: 43cf75d96409 ("exit: panic before exit_mm() on global init
> > exit")
> > Link:
> > https://lore.kernel.org/all/20191219104223.xvk6ppfogoxrgmw6@wittgenstein/
> > Signed-off-by: Tze-nan Wu <Tze-nan.Wu@mediatek.com>
> > ---
> > 
> > I am also wondering if this patch is reasonable?
> > 
> > From my perspective, there are two reasons not to trigger such
> > panic
> > during kernel_power_off() or kernel_restart():
> >   1. It is not worthwhile to interrupt kernel_power_off() by a
> > panic
> >      resulted from userspace instability.
> >   2. The panic in do_exit() was originally designed to ensure a
> > usable
> >      coredump if the last thread of the global init process exited.
> >        However, capture a coredump triggered by userspace crash
> > after
> >      kernel_power_off() seems not particularly useful, in my
> > opinion.
> > 
> > In certain scenarios, a kernel module may need to directly power
> > off
> > from kernel space to protect hardware (e.g., thermal protection).
> > In my opinion, rather than causing a panic during
> > kernel_power_off(),
> > it sounds better to allow the device to complete its power-off
> > process.
> > 
> > Appreciate for any comment on this, if there's any better way to
> > handle this panic, please point me out.
> > 
> > ---
> >  kernel/exit.c | 14 ++++++++++----
> >  1 file changed, 10 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/exit.c b/kernel/exit.c
> > index 1dcddfe537ee..23cb6b42a1f1 100644
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -901,11 +901,17 @@ void __noreturn do_exit(long code)
> >       if (group_dead) {
> >               /*
> >                * If the last thread of global init has exited,
> > panic
> > -              * immediately to get a useable coredump.
> > +              * immediately to get a usable coredump, except when
> > the
> > +              * device is currently powering off or restarting.
> >                */
> > -             if (unlikely(is_global_init(tsk)))
> > -                     panic("Attempted to kill init!
> > exitcode=0x%08x\n",
> > -                             tsk->signal->group_exit_code ?:
> > (int)code);
> > +             if (unlikely(is_global_init(tsk))) {
> > +                     if (system_state != SYSTEM_POWER_OFF &&
> > +                         system_state != SYSTEM_RESTART)
> > +                             panic("Attempted to kill init!
> > exitcode=0x%08x\n",
> > +                                   tsk->signal->group_exit_code ?:
> > (int)code);
> > +                     WARN(1, "Attempted to kill init!
> > exitcode=0x%08x\n",
> > +                          tsk->signal->group_exit_code ?:
> > (int)code);
> > +             }
> > 
> >  #ifdef CONFIG_POSIX_TIMERS
> >               hrtimer_cancel(&tsk->signal->real_timer);
> > --
> > 2.45.2
> > 
>