Peter Zijlstra
2023-Jan-26 09:28 UTC
[PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled
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 at infradead.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 at infradead.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;