From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v3 05/22] rcu: Make RCU IRQ enter/exit functions rely on in_nmi() Date: Wed, 19 Feb 2020 18:03:04 +0100 Message-ID: <20200219170304.GG14946@hirez.programming.kicks-ass.net> References: <20200219144724.800607165@infradead.org> <20200219150744.661923520@infradead.org> <20200219163156.GY2935@paulmck-ThinkPad-P72> <20200219163700.GK18400@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from merlin.infradead.org ([205.233.59.134]:37258 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726518AbgBSRDd (ORCPT ); Wed, 19 Feb 2020 12:03:33 -0500 Content-Disposition: inline In-Reply-To: <20200219163700.GK18400@hirez.programming.kicks-ass.net> Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, rostedt@goodmis.org, mingo@kernel.org, joel@joelfernandes.org, gregkh@linuxfoundation.org, gustavo@embeddedor.com, tglx@linutronix.de, josh@joshtriplett.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, luto@kernel.org, tony.luck@intel.com, frederic@kernel.org, dan.carpenter@oracle.com, mhiramat@kernel.org 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: > > 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 ;-) Having gone over the old thread, I ended up with the below. Anyone holler if I got it wrong somehow. --- Subject: rcu: Provide comment for NOKPROBE() on rcu_nmi_enter() From: Steven Rostedt From: Steven Rostedt 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 Reviewed-by: Masami Hiramatsu Signed-off-by: Paul E. McKenney Signed-off-by: Peter Zijlstra (Intel) --- kernel/rcu/tree.c | 8 ++++++++ 1 file changed, 8 insertions(+) --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -842,6 +842,14 @@ 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); /**