Re: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled

[Peter Zijlstra]

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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
---
 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;