Date:   Sat, 28 Jan 2023 11:12:02 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Steven Rostedt <rostedt@...dmis.org>, mingo@...nel.org,
        will@...nel.org, boqun.feng@...il.com, tglx@...utronix.de,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, seanjc@...gle.com, pbonzini@...hat.com,
        jgross@...e.com, srivatsa@...il.mit.edu, amakhalov@...are.com,
        pv-drivers@...are.com, mhiramat@...nel.org, wanpengli@...cent.com,
        vkuznets@...hat.com, boris.ostrovsky@...cle.com, rafael@...nel.org,
        daniel.lezcano@...aro.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        linux-trace-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU
 is disabled

On Thu, Jan 26, 2023 at 10:28:51AM +0100, Peter Zijlstra wrote:
> On Wed, Jan 25, 2023 at 10:46:58AM -0800, Paul E. McKenney wrote:
> 
> > > Ofc. Paul might have an opinion on this glorious bodge ;-)
> > 
> > For some definition of the word "glorious", to be sure.  ;-)
> > 
> > Am I correct that you have two things happening here?  (1) Preventing
> > trace recursion and (2) forcing RCU to pay attention when needed.
> 
> Mostly just (1), we're in an error situation, I'm not too worried about
> (2).
> 
> > I cannot resist pointing out that you have re-invented RCU_NONIDLE(),
> > though avoiding much of the overhead when not needed.  ;-)
> 
> Yeah, this was the absolute minimal bodge I could come up with that
> shuts up the rcu_derefence warning thing.
> 
> > I would have objections if this ever leaks out onto a non-error code path.
> 
> Agreed.
> 
> > There are things that need doing when RCU starts and stops watching,
> > and this approach omits those things.  Which again is OK in this case,
> > where this code is only ever executed when something is already broken,
> > but definitely *not* OK when things are not already broken.
> 
> And agreed.
> 
> Current version of the bodge looks like so (will repost the whole series
> a little later today).
> 
> I managed to tickle the recursion so that it was a test-case for the
> stack guard...
> 
> With this on, it prints just the one WARN and lives.
> 
> ---
> Subject: bug: Disable rcu_is_watching() during WARN/BUG
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Wed Jan 25 13:57:49 CET 2023
> 
> In order to avoid WARN/BUG from generating nested or even recursive
> warnings, force rcu_is_watching() true during
> WARN/lockdep_rcu_suspicious().
> 
> Notably things like unwinding the stack can trigger rcu_dereference()
> warnings, which then triggers more unwinding which then triggers more
> warnings etc..
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

>From an RCU perspective:

Acked-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  include/linux/context_tracking.h |   27 +++++++++++++++++++++++++++
>  kernel/locking/lockdep.c         |    3 +++
>  kernel/panic.c                   |    5 +++++
>  lib/bug.c                        |   15 ++++++++++++++-
>  4 files changed, 49 insertions(+), 1 deletion(-)
> 
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -130,9 +130,36 @@ static __always_inline unsigned long ct_
>  	return arch_atomic_add_return(incby, this_cpu_ptr(&context_tracking.state));
>  }
>  
> +static __always_inline bool warn_rcu_enter(void)
> +{
> +	bool ret = false;
> +
> +	/*
> +	 * Horrible hack to shut up recursive RCU isn't watching fail since
> +	 * lots of the actual reporting also relies on RCU.
> +	 */
> +	preempt_disable_notrace();
> +	if (rcu_dynticks_curr_cpu_in_eqs()) {
> +		ret = true;
> +		ct_state_inc(RCU_DYNTICKS_IDX);
> +	}
> +
> +	return ret;
> +}
> +
> +static __always_inline void warn_rcu_exit(bool rcu)
> +{
> +	if (rcu)
> +		ct_state_inc(RCU_DYNTICKS_IDX);
> +	preempt_enable_notrace();
> +}
> +
>  #else
>  static inline void ct_idle_enter(void) { }
>  static inline void ct_idle_exit(void) { }
> +
> +static __always_inline bool warn_rcu_enter(void) { return false; }
> +static __always_inline void warn_rcu_exit(bool rcu) { }
>  #endif /* !CONFIG_CONTEXT_TRACKING_IDLE */
>  
>  #endif
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -55,6 +55,7 @@
>  #include <linux/rcupdate.h>
>  #include <linux/kprobes.h>
>  #include <linux/lockdep.h>
> +#include <linux/context_tracking.h>
>  
>  #include <asm/sections.h>
>  
> @@ -6555,6 +6556,7 @@ void lockdep_rcu_suspicious(const char *
>  {
>  	struct task_struct *curr = current;
>  	int dl = READ_ONCE(debug_locks);
> +	bool rcu = warn_rcu_enter();
>  
>  	/* Note: the following can be executed concurrently, so be careful. */
>  	pr_warn("\n");
> @@ -6595,5 +6597,6 @@ void lockdep_rcu_suspicious(const char *
>  	lockdep_print_held_locks(curr);
>  	pr_warn("\nstack backtrace:\n");
>  	dump_stack();
> +	warn_rcu_exit(rcu);
>  }
>  EXPORT_SYMBOL_GPL(lockdep_rcu_suspicious);
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -34,6 +34,7 @@
>  #include <linux/ratelimit.h>
>  #include <linux/debugfs.h>
>  #include <linux/sysfs.h>
> +#include <linux/context_tracking.h>
>  #include <trace/events/error_report.h>
>  #include <asm/sections.h>
>  
> @@ -679,6 +680,7 @@ void __warn(const char *file, int line,
>  void warn_slowpath_fmt(const char *file, int line, unsigned taint,
>  		       const char *fmt, ...)
>  {
> +	bool rcu = warn_rcu_enter();
>  	struct warn_args args;
>  
>  	pr_warn(CUT_HERE);
> @@ -693,11 +695,13 @@ void warn_slowpath_fmt(const char *file,
>  	va_start(args.args, fmt);
>  	__warn(file, line, __builtin_return_address(0), taint, NULL, &args);
>  	va_end(args.args);
> +	warn_rcu_exit(rcu);
>  }
>  EXPORT_SYMBOL(warn_slowpath_fmt);
>  #else
>  void __warn_printk(const char *fmt, ...)
>  {
> +	bool rcu = warn_rcu_enter();
>  	va_list args;
>  
>  	pr_warn(CUT_HERE);
> @@ -705,6 +709,7 @@ void __warn_printk(const char *fmt, ...)
>  	va_start(args, fmt);
>  	vprintk(fmt, args);
>  	va_end(args);
> +	warn_rcu_exit(rcu);
>  }
>  EXPORT_SYMBOL(__warn_printk);
>  #endif
> --- a/lib/bug.c
> +++ b/lib/bug.c
> @@ -47,6 +47,7 @@
>  #include <linux/sched.h>
>  #include <linux/rculist.h>
>  #include <linux/ftrace.h>
> +#include <linux/context_tracking.h>
>  
>  extern struct bug_entry __start___bug_table[], __stop___bug_table[];
>  
> @@ -153,7 +154,7 @@ struct bug_entry *find_bug(unsigned long
>  	return module_find_bug(bugaddr);
>  }
>  
> -enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
> +static enum bug_trap_type __report_bug(unsigned long bugaddr, struct pt_regs *regs)
>  {
>  	struct bug_entry *bug;
>  	const char *file;
> @@ -209,6 +210,18 @@ enum bug_trap_type report_bug(unsigned l
>  	return BUG_TRAP_TYPE_BUG;
>  }
>  
> +enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
> +{
> +	enum bug_trap_type ret;
> +	bool rcu = false;
> +
> +	rcu = warn_rcu_enter();
> +	ret = __report_bug(bugaddr, regs);
> +	warn_rcu_exit(rcu);
> +
> +	return ret;
> +}
> +
>  static void clear_once_table(struct bug_entry *start, struct bug_entry *end)
>  {
>  	struct bug_entry *bug;

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