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Message-ID: <20230128191202.GB2948950@paulmck-ThinkPad-P17-Gen-1> 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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