Re: [PATCH 0/2] panic/printk/x86: Prevent some more printk-related deadlocks in panic()

[Konstantin Khlebnikov]

On Tue, Jul 16, 2019 at 10:28 AM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> Hi,
>
> I have found some spare duct tape and wrapped some more printk-related
> deadlocks in panic().
>
> More seriously, someone reported a deadlock in panic(). Some non-trivial
> debugging pointed out a problem with the following combination:
>
>      + x86_64 architecture
>      + panic()
>      + pstore configured as message dumper (kmsg_dump())
>      + crash kernel configured
>      + crash_kexec_post_notifiers
>
> In this case, CPUs are stopped by crash_smp_send_stop(). It uses
> NMI but it does not modify cpu_online_mask. Therefore logbuf_lock
> might stay locked, see 2nd patch for more details.
>
> The above is a real corner case. But similar problem seems to be
> even in the common situations on architectures that do not use
> NMI in smp_send_stop() as a fallback, see 1st patch.
>
> Back to the duct tape. I hope that we will get rid of these problems
> with the lockless printk ringbuffer rather sooner than later.
> But it still might take some time. And the two fixes might be
> useful also for stable kernels.

Looks good.

Reviewed-by: Konstantin Khebnikov <khlebnikov@xxxxxxxxxxxxxx>

>
>
> Petr Mladek (2):
>   printk/panic: Access the main printk log in panic() only when safe
>   printk/panic/x86: Allow to access printk log buffer after
>     crash_smp_send_stop()
>
>  arch/x86/kernel/crash.c     |  6 +++++-
>  include/linux/printk.h      |  6 ++++++
>  kernel/panic.c              | 49 +++++++++++++++++++++++++++------------------
>  kernel/printk/printk_safe.c | 37 ++++++++++++++++++++++------------
>  4 files changed, 65 insertions(+), 33 deletions(-)
>
> --
> 2.16.4
>