linux-kernel - Re: [PATCH v3 05/22] rcu: Make RCU IRQ enter/exit functions rely on in

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Message-ID: <20200219164544.GD2935@paulmck-ThinkPad-P72>
Date:   Wed, 19 Feb 2020 08:45:44 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, frederic@...nel.org,
        dan.carpenter@...cle.com, mhiramat@...nel.org
Subject: Re: [PATCH v3 05/22] rcu: Make RCU IRQ enter/exit functions rely on
 in_nmi()

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@...nel.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@...nel.org>
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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