printk: nbcon: Allow reacquire during panic

If a console printer is interrupted during panic, it will never
be able to reacquire ownership in order to perform and cleanup.
That in itself is not a problem, since the non-panic CPU will
simply quiesce in an endless loop within nbcon_reacquire_nobuf().

However, in this state, platforms that do not support a true NMI
to interrupt the quiesced CPU will not be able to shutdown that
CPU from within panic(). This then causes problems for such as
being unable to load and run a kdump kernel.

Fix this by allowing non-panic CPUs to reacquire ownership using
a direct acquire. Then the non-panic CPUs can successfullyl exit
the nbcon_reacquire_nobuf() loop and the console driver can
perform any necessary cleanup. But more importantly, the CPU is
no longer quiesced and is free to process any interrupts
necessary for panic() to shutdown the CPU.

All other forms of acquire are still not allowed for non-panic
CPUs since it is safer to have them avoid gaining console
ownership that is not strictly necessary.

Reported-by: Michael Kelley <mhklinux@outlook.com>
Closes: https://lore.kernel.org/r/SN6PR02MB4157A4C5E8CB219A75263A17D46DA@SN6PR02MB4157.namprd02.prod.outlook.com
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Link: https://patch.msgid.link/20250606185549.900611-1-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

0 files changed, 0 insertions, 0 deletions