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

Paul E. McKenney
2020-02-19T16:45:47Z

On Wed, Feb 19, 2020 at 05:37:00PM +0100, Peter Zijlstra wrote:
> 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 ;-)

There was a lot of effort spent on it, to be sure. ;-) ;-) ;-)

Thanx, Paul