Re: [PATCH printk v4 4/6] printk: remove NMI tracking

John Ogness
2021-07-21T13:23:49Z

On 2021-07-21, Petr Mladek <pmladek@suse.com> wrote:
> On Wed 2021-07-21 14:52:15, John Ogness wrote:
>> On 2021-07-21, Petr Mladek <pmladek@suse.com> wrote:
>> >> --- a/kernel/trace/trace.c
>> >> +++ b/kernel/trace/trace.c
>> >> @@ -9647,7 +9647,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
>> >> tracing_off();
>> >>
>> >> local_irq_save(flags);
>> >> - printk_nmi_direct_enter();
>> >> + printk_deferred_enter();
>> >
>> > I would prefer to do not manipulate the printk context here anymore,
>> > as it was done in v3.
>> >
>> > printk_nmi_direct_enter() was added here by the commit the commit
>> > 03fc7f9c99c1e7ae2925d4 ("printk/nmi: Prevent deadlock when
>> > accessing the main log buffer in NMI"). It was _not_ about console
>> > handling. The reason was to modify the default behavior under NMI
>> > and store the messages directly into the main log buffer.
>> >
>> > When I think about it. The original fix was not correct. We should have
>> > modified the context only when ftrace_dump() was really called under NMI:
>> >
>> > if (in_nmi())
>> > printk_nmi_direct_enter();
>> >
>> > By other words. We should try to show the messages on the console
>> > when ftrace_dump()/panic() is not called from NMI. It will help
>> > to see all messages even when the ftrace buffers are bigger
>> > than printk() ones.
>> >
>> > And we do not need any special handling here for NMI. vprintk()
>> > in printk/printk_safe.c will do the right thing for us.
>>
>> Agreed. We need to mention this behavior change in the commit
>> message. Perhaps this as the commit message:
>>
>> All NMI contexts are handled the same as the safe context: store the
>> message and defer printing. There is no need to have special NMI
>> context tracking for this. Using in_nmi() is enough.
>>
>> There are several parts of the kernel that are manually calling into
>> the printk NMI context tracking in order to cause general printk
>> deferred printing:
>>
>> arch/arm/kernel/smp.c
>> arch/powerpc/kexec/crash.c
>> kernel/trace/trace.c
>>
>> For arm/kernel/smp.c and powerpc/kexec/crash.c, provide a new
>> function pair printk_deferred_enter/exit that explicitly achieves the
>> same objective.
>>
>> For ftrace, remove general printk deferring. This general deferrment
>> was added in commit 03fc7f9c99c1 ("printk/nmi: Prevent deadlock when
>> accessing the main log buffer in NMI"), but really should have only
>> been deferred when in NMI context. Since vprintk() now checks for
>> NMI context when deciding to defer, ftrace does not need any special
>> handling.
>
> I would make it less focused on the deferring part and try to explain
> the original purpose here, something like:
>
> "For ftrace, remove the printk context manipulation completely. It was
> added in commit 03fc7f9c99c1 ("printk/nmi: Prevent deadlock when
> accessing the main log buffer in NMI"). The purpose was to enforce
> storing messages directly into the ring buffer even in NMI context.
> It really should have only modified the behavior in NMI context.
> There is no need for a special behavior any longer. All messages are
> always stored directly now. The console deferring is handled
> transparently in vprintk()."

Your wording is OK for me.

John Ogness