Re: [PATCH v3 05/22] rcu: Make RCU IRQ enter/exit functions rely on in_nmi()

Peter Zijlstra
2020-02-19T16:37:31Z

On Wed, Feb 19, 2020 at 08:31:56AM -0800, Paul E. McKenney wrote:
> On Wed, Feb 19, 2020 at 03:47:29PM +0100, Peter Zijlstra wrote:
> > From: Paul E. McKenney <paulmck@kernel.org>
> >
> > The rcu_nmi_enter_common() and rcu_nmi_exit_common() functions take an
> > "irq" parameter that indicates whether these functions are invoked from
> > an irq handler (irq==true) or an NMI handler (irq==false). However,
> > recent changes have applied notrace to a few critical functions such
> > that rcu_nmi_enter_common() and rcu_nmi_exit_common() many now rely
> > on in_nmi(). Note that in_nmi() works no differently than before,
> > but rather that tracing is now prohibited in code regions where in_nmi()
> > would incorrectly report NMI state.
> >
> > This commit therefore removes the "irq" parameter and inlines
> > rcu_nmi_enter_common() and rcu_nmi_exit_common() into rcu_nmi_enter()
> > and rcu_nmi_exit(), respectively.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>
> Again, thank you.
>
> Would you like to also take the added comment for NOKPROBE_SYMBOL(),
> or would you prefer that I carry that separately? (I dropped it for
> now to avoid the conflict with the patch below.)
>
> Here is the latest version of that comment, posted by Steve Rostedt.
>
> Thanx, Paul
>
> /*
> * All functions called in the breakpoint trap handler (e.g. do_int3()
> * on x86), must not allow kprobes until the kprobe breakpoint handler
> * is called, otherwise it can cause an infinite recursion.
> * On some archs, rcu_nmi_enter() is called in the breakpoint handler
> * before the kprobe breakpoint handler is called, thus it must be
> * marked as NOKPROBE.
> */

Oh right, let me stick that in a separate patch. Best we not loose that
I suppose ;-)