On Mon, Feb 17, 2020 at 05:55:19PM +0900, Masami Hiramatsu wrote: > On Sat, 15 Feb 2020 06:59:34 -0800 > "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > > > On Fri, Feb 14, 2020 at 03:19:06PM +0900, Masami Hiramatsu wrote: > > > On Thu, 13 Feb 2020 14:39:18 -0800 > > > "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > > > > > > > On Thu, Feb 13, 2020 at 05:04:51PM -0500, Steven Rostedt wrote: > > > > > On Thu, 13 Feb 2020 13:50:04 -0800 > > > > > "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > > > > > > > > > > > On Thu, Feb 13, 2020 at 04:38:25PM -0500, Steven Rostedt wrote: > > > > > > > [ Added Masami ] > > > > > > > > > > > > > > On Thu, 13 Feb 2020 16:19:30 -0500 > > > > > > > Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > > > On Thu, Feb 13, 2020 at 12:54:42PM -0800, Paul E. McKenney wrote: > > > > > > > > > On Thu, Feb 13, 2020 at 03:44:44PM -0500, Joel Fernandes wrote: > > > > > > > > > > On Thu, Feb 13, 2020 at 10:56:12AM -0800, Paul E. McKenney wrote: > > > > > > > > > > [...] > > > > > > > > > > > > > It might well be that I could make these functions be NMI-safe, but > > > > > > > > > > > > > rcu_prepare_for_idle() in particular would be a bit ugly at best. > > > > > > > > > > > > > So, before looking into that, I have a question. Given these proposed > > > > > > > > > > > > > changes, will rcu_nmi_exit_common() and rcu_nmi_enter_common() be able > > > > > > > > > > > > > to just use in_nmi()? > > > > > > > > > > > > > > > > > > > > > > > > That _should_ already be the case today. That is, if we end up in a > > > > > > > > > > > > tracer and in_nmi() is unreliable we're already screwed anyway. > > > > > > > > > > > > > > > > > > > > > > So something like this, then? This is untested, probably doesn't even > > > > > > > > > > > build, and could use some careful review from both Peter and Steve, > > > > > > > > > > > at least. As in the below is the second version of the patch, the first > > > > > > > > > > > having been missing a couple of important "!" characters. > > > > > > > > > > > > > > > > > > > > I removed the static from rcu_nmi_enter()/exit() as it is called from > > > > > > > > > > outside, that makes it build now. Updated below is Paul's diff. I also added > > > > > > > > > > NOKPROBE_SYMBOL() to rcu_nmi_exit() to match rcu_nmi_enter() since it seemed > > > > > > > > > > asymmetric. > > > > > > > > > > > > > > > > > > My compiler complained about the static and the __always_inline, so I > > > > > > > > > fixed those. But please help me out on adding the NOKPROBE_SYMBOL() > > > > > > > > > to rcu_nmi_exit(). What bad thing happens if we leave this on only > > > > > > > > > rcu_nmi_enter()? > > > > > > > > > > > > > > > > It seemed odd to me we were not allowing kprobe on the rcu_nmi_enter() but > > > > > > > > allowing it on exit (from a code reading standpoint) so my reaction was to > > > > > > > > add it to both, but we could probably keep that as a separate > > > > > > > > patch/discussion since it is slightly unrelated to the patch.. Sorry to > > > > > > > > confuse the topic. > > > > > > > > > > > > > > > > > > > > > > rcu_nmi_enter() was marked NOKPROBE or other reasons. See commit > > > > > > > c13324a505c77 ("x86/kprobes: Prohibit probing on functions before > > > > > > > kprobe_int3_handler()") > > > > > > > > > > > > > > The issue was that we must not allow anything in do_int3() call kprobe > > > > > > > code before kprobe_int3_handler() is called. Because ist_enter() (in > > > > > > > do_int3()) calls rcu_nmi_enter() it had to be marked NOKPROBE. It had > > > > > > > nothing to do with it being RCU nor NMI, but because it was simply > > > > > > > called in do_int3(). > > > > > > > > > > > > > > Thus, there's no reason to make rcu_nmi_exit() NOKPROBE. But a commont > > > > > > > to why rcu_nmi_enter() would probably be useful, like below: > > > > > > > > > > > > Thank you, Steve! Could I please have your Signed-off-by for this? > > > > > > > > > > Sure, but it was untested ;-) > > > > > > > > No problem! I will fire up rcutorture on it. ;-) > > > > > > > > But experience indicates that you cannot even make a joke around here. > > > > There is probably already someone out there somewhere building a > > > > comment-checker based on deep semantic analysis and machine learning. :-/ > > > > > > > > > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > > > > > > > > > > I'd like a Reviewed-by from Masami though. > > > > > > > > Sounds good! Masami, would you be willing to review? > > > > > > Yes, the functions before calling kprobe_int3_handler() must not > > > be kprobed. It can cause an infinite recursive int3 trapping. > > > > > > Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx> > > > > Thank you both! > > > > Like this? > > > > Thanx, Paul > > > > This is good to me. Thank you for looking it over! (I already have your > BTW, if you consider the x86 specific code is in the generic file, > we can move NOKPROBE_SYMBOL() in arch/x86/kernel/traps.c. > (Sorry, I've hit this idea right now) Might this affect other architectures with NMIs and probe-like things? If so, it might make sense to leave it where it is. Thanx, Paul > Thank you, > > > ------------------------------------------------------------------------ > > > > commit 1817fdc8f4e4bd18c76305c9b937fb0dccbb1583 > > Author: Steven Rostedt <rostedt@xxxxxxxxxxx> > > Date: Sat Feb 15 06:54:50 2020 -0800 > > > > rcu: Provide comment for NOKPROBE() on rcu_nmi_enter() > > > > The rcu_nmi_enter() function was marked NOKPROBE() by commit > > c13324a505c77 ("x86/kprobes: Prohibit probing on functions before > > kprobe_int3_handler()") because the do_int3() call kprobe code must > > not be invoked before kprobe_int3_handler() is called. It turns out > > that ist_enter() (in do_int3()) calls rcu_nmi_enter(), hence the > > marking NOKPROBE() being added to rcu_nmi_enter(). > > > > This commit therefore adds a comment documenting this line of reasoning. > > > > Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx> > > Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index 132b53e..4a885af 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -835,6 +835,12 @@ void rcu_nmi_enter(void) > > rdp->dynticks_nmi_nesting + incby); > > barrier(); > > } > > +/* > > + * On x86, All functions in do_int3() must be marked NOKPROBE before > > + * kprobe_int3_handler() is called. ist_enter() which is called in do_int3() > > + * before kprobe_int3_handle() happens to call rcu_nmi_enter() which means > > + * that rcu_nmi_enter() must be marked NOKRPOBE. > > + */ > > NOKPROBE_SYMBOL(rcu_nmi_enter); > > > > /** > > > -- > Masami Hiramatsu <mhiramat@xxxxxxxxxx>