On Thu 2018-09-06 11:31:51, Sergey Senozhatsky wrote:
> Hello,
>
> On (09/05/18 21:33), Steven Rostedt wrote:
> > do_idle {
> >
> > [interrupts enabled]
> >
> > <interrupt> [interrupts disabled]
> > TRACE_IRQS_OFF [lockdep says irqs off]
> > [...]
> > TRACE_IRQS_IRET
> > test if pt_regs say return to interrupts enabled [yes]
> > TRACE_IRQS_ON [lockdep says irqs are on]
> >
> > <nmi>
> > nmi_enter() {
> > printk_nmi_enter() [traced by ftrace]
> > [ hit ftrace breakpoint ]
> > <breakpoint exception>
> > TRACE_IRQS_OFF [lockdep says irqs off]
> > [...]
> > TRACE_IRQS_IRET [return from breakpoint]
> > test if pt_regs say interrupts enabled [no]
> > [iret back to interrupt]
> > [iret back to code]
> >
> > tick_nohz_idle_enter() {
> >
> > lockdep_assert_irqs_enabled() [lockdep say no!]
> >
> > Although interrupts are indeed enabled, lockdep thinks it is not, and since
> > we now do asserts via lockdep, it gives a false warning. The issue here is
> > that printk_nmi_enter() is called before lockdep_off(), which disables
> > lockdep (for this reason) in NMIs. By simply not allowing ftrace to see
> > printk_nmi_enter() (via notrace annotation) we keep lockdep from getting
> > confused.
>
> Great catch and I like the patch!
>
> Indeed, with printk_nmi we changed the "everything that nmi_enter does
> should happen after lockdep_off" to "everything that nmi_enter does should
> happen after printk_nmi_enter" // +similar change to nmi_exit.
>
> An alternative option, thus, could be re-instating back the rule that
> lockdep_off/on should be the first and the last thing we do in
> nmi_enter/nmi_exit. E.g.
>
> nmi_enter()
> lockdep_off();
> printk_nmi_enter();
>
> nmi_exit()
> printk_nmi_exit();
> lockdep_on();
Yup, this would help here as well.
> I guess that we can keep printk_nmi_enter/printk_nmi_exit at the top
> and at the bottom of nmi_enter/nmi_exit correspondingly just in case
> if lockdep_off/lockdep_on sometime in the future starts invoking printk(),
> which would deadlock us. Hence
Yes.
In general, it does not looks safe to allow tracing functions
that are called before ftrace_nmi_enter().
> Acked-by: Sergey Senozhatsky <sergey.senozhat...@gmail.com>
Therefore I am all for Steven's patch.
Acked-by: Petr Mladek <pmla...@suse.com>
Best Regards,
Petr
