Messages in this thread | | | Subject | Re: [PATCH v2] kernel/printk: prevent deadlock at unexpected call kmsg_dump in NMI context | From | Konstantin Khlebnikov <> | Date | Tue, 16 Jul 2019 10:55:14 +0300 |
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On 16.07.2019 10:41, Petr Mladek wrote: > 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: > > + oops_end() > + oops_exit() > + kmsg_dump(KMSG_DUMP_OOPS) > > 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?
Yep. Under 'oops in nmi context' I mean exactly that case.
> > Otherwise, the patch looks good to me. I would just mention > the above scenario if it is correct. > > Best Regards, > Petr > >> This patch catches kmsg_dump() called in NMI context except panic and >> prints warning once. >> >> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> >> 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) >> return; >> >>
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