Re: [PATCH v2] kernel/printk: prevent deadlock at unexpected call kmsg_dump in NMI context
From: Petr Mladek
Date: Tue Jul 16 2019 - 03:41:08 EST
On Mon 2019-07-15 11:04:55, Konstantin Khlebnikov wrote:
> Kernel message dumper - function kmsg_dump() is called on various oops or
> panic paths which could happen in unpredictable context including NMI.
> Panic in NMI is handled especially by stopping all other cpus with
> smp_send_stop() and busting locks in printk_safe_flush_on_panic().
> Other less-fatal cases shouldn't happen in NMI and cannot be handled.
> But this might happen for example on oops in nmi context. In this case
> dumper could deadlock on lockbuf_lock or break internal structures.
If I get it correctly than this patch could really prevent a deadlock
in at least:
If it is called in NMI, it should end up with panic(). Then the dump
will be called later after stopping CPUs...
Or am I wrong?
Otherwise, the patch looks good to me. I would just mention
the above scenario if it is correct.
> This patch catches kmsg_dump() called in NMI context except panic and
> prints warning once.
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/156294329676.1745.2620297516210526183.stgit@buzz/ (v1)
> kernel/printk/printk.c | 7 +++++++
> 1 file changed, 7 insertions(+)
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 1888f6a3b694..e711f64a1843 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3104,6 +3104,13 @@ void kmsg_dump(enum kmsg_dump_reason reason)
> struct kmsg_dumper *dumper;
> unsigned long flags;
> + /*
> + * In NMI context only panic could be handled safely:
> + * it stops other cpus and busts logbuf lock.
> + */
> + if (WARN_ON_ONCE(reason != KMSG_DUMP_PANIC && in_nmi()))
> + return;
> if ((reason > KMSG_DUMP_OOPS) && !always_kmsg_dump)