Re: [PATCH v4 08/27] rcu/kprobes: Comment why rcu_nmi_enter() is marked NOKPROBE

[Frederic Weisbecker]

On Fri, Feb 21, 2020 at 02:34:24PM +0100, Peter Zijlstra 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.
> 
> Reported-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Acked-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20200213163800.5c51a5f1@xxxxxxxxxxxxxxxxxx

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>