Messages in this thread | | | Date | Thu, 6 Sep 2018 11:04:54 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH] printk/tracing: Do not trace printk_nmi_enter() |
| |
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.senozhatsky@gmail.com>
Therefore I am all for Steven's patch.
Acked-by: Petr Mladek <pmladek@suse.com>
Best Regards, Petr
| |