On Wed, Feb 19, 2020 at 12:16 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > > It's confusing that rcu_nmi_enter() is marked NOKPROBE and > rcu_nmi_exit() is not. One may think that the exit needs to be marked > for the same reason the enter is, as rcu_nmi_exit() reverts the RCU > state back to what it was before rcu_nmi_enter(). But the reason has > nothing to do with the state of RCU. > > The breakpoint handler (int3 on x86) must not have any kprobe on it > until the kprobe handler is called. Otherwise, it can cause an infinite > recursion and crash the machine. It just so happens that > rcu_nmi_enter() is called by the int3 handler before the kprobe handler > can run, and therefore needs to be marked as NOKPROBE. > > Comment this to remove the confusion to why rcu_nmi_enter() is marked > NOKPROBE but rcu_nmi_exit() is not. > > Link: https://lore.kernel.org/r/20200213163800.5c51a5f1@xxxxxxxxxxxxxxxxxx > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> Reported-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Acked-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> thanks, - Joel > --- > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 1694a6b57ad8..ada7b2b638fb 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -846,6 +846,14 @@ void rcu_nmi_enter(void) > { > rcu_nmi_enter_common(false); > } > +/* > + * 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. > + */ > NOKPROBE_SYMBOL(rcu_nmi_enter); > > /**
