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

On 2021-07-21, Petr Mladek [off-list ref] wrote:

quoted

--- 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.

Signed-off-by: John Ogness <john.ogness@linutronix.de>