Messages in this thread | | | Date | Wed, 13 May 2020 14:36:21 +1000 | From | Nicholas Piggin <> | Subject | Re: [PATCH v2 1/1] powerpc/crash: Use NMI context for printk when starting to crash |
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Excerpts from Leonardo Bras's message of May 13, 2020 7:45 am: > Currently, if printk lock (logbuf_lock) is held by other thread during > crash, there is a chance of deadlocking the crash on next printk, and > blocking a possibly desired kdump. > > At the start of default_machine_crash_shutdown, make printk enter > NMI context, as it will use per-cpu buffers to store the message, > and avoid locking logbuf_lock.
printk_nmi_enter is used in one other place outside nmi_enter.
Is there a different/better way to handle this? What do other architectures do?
Other subsystems get put into an nmi-mode when we call nmi_enter (lockdep, ftrace, rcu etc). It seems like those would be useful for similar reasons, so at least explaining why that is not used in a comment would be good.
Aside from that, I welcome any effort to make our crashes more reliable so thanks for working on this stuff.
Thanks, Nick
> > Suggested-by: Michael Ellerman <mpe@ellerman.id.au> > Signed-off-by: Leonardo Bras <leobras.c@gmail.com> > > --- > Changes since v1: > - Added in-code comment explaining the need of context change > - Function moved to the start of default_machine_crash_shutdown, > to avoid locking any printk on crashing routine. > - Title was 'Use NMI context for printk after crashing other CPUs' > > --- > arch/powerpc/kexec/crash.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c > index d488311efab1..c9a889880214 100644 > --- a/arch/powerpc/kexec/crash.c > +++ b/arch/powerpc/kexec/crash.c > @@ -311,6 +311,9 @@ void default_machine_crash_shutdown(struct pt_regs *regs) > unsigned int i; > int (*old_handler)(struct pt_regs *regs); > > + /* Avoid hardlocking with irresponsive CPU holding logbuf_lock */ > + printk_nmi_enter(); > + > /* > * This function is only called after the system > * has panicked or is otherwise in a critical state. > -- > 2.25.4 > >
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