Re: [PATCH] tracing: generate RCU warnings even when tracepoints are disabled

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Aug 07, 2014 at 10:52:04AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> Dave Jones reported seeing a bug from one of my TLB tracepoints:
> 
> 	http://lkml.kernel.org/r/20140806181801.GA4605@redhat.com
> 
> I've been running these patches for months and never saw this.
> But, a big chunk of my testing, especially with all the debugging
> enabled, was in a vm where intel_idle doesn't work.  On the
> systems where I was using intel_idle, I never had lockdep enabled
> and this tracepoint on at the same time.
> 
> This patch ensures that whenever we have lockdep available, we do
> _some_ RCU activity at the site of the tracepoint, despite
> whether the tracepoint's condition matches or even if the
> tracepoint itself is completely disabled.  This is a bit of a
> hack, but it is pretty self-contained.
> 
> I confirmed that with this patch plus lockdep I get the same
> splat as Dave Jones did, but without enabling the tracepoint
> explicitly.
> 
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Dave Jones <davej@redhat.com>,
> Cc: paulmck@linux.vnet.ibm.com
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ingo Molnar <mingo@redhat.com>

Looks good to me, but I must defer to Steven on this one.

							Thanx, Paul

> ---
> 
>  b/include/linux/tracepoint.h |   11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff -puN include/linux/tracepoint.h~easier-rcu-splat include/linux/tracepoint.h
> --- a/include/linux/tracepoint.h~easier-rcu-splat	2014-08-07 10:29:32.701217956 -0700
> +++ b/include/linux/tracepoint.h	2014-08-07 10:40:35.244627014 -0700
> @@ -157,6 +157,12 @@ extern void syscall_unregfunc(void);
>   * Make sure the alignment of the structure in the __tracepoints section will
>   * not add unwanted padding between the beginning of the section and the
>   * structure. Force alignment to the same alignment as the section start.
> + *
> + * When lockdep is enabled, we make sure to always do the RCU portions of
> + * the tracepoint code, regardless of whether tracing is on or we match the
> + * condition.  This lets us find RCU issues triggered with tracepoints even
> + * when this tracepoint is off.  This code has no purpose other than poking
> + * RCU a bit.
>   */
>  #define __DECLARE_TRACE(name, proto, args, cond, data_proto, data_args) \
>  	extern struct tracepoint __tracepoint_##name;			\
> @@ -167,6 +173,11 @@ extern void syscall_unregfunc(void);
>  				TP_PROTO(data_proto),			\
>  				TP_ARGS(data_args),			\
>  				TP_CONDITION(cond),,);			\
> +		if (IS_ENABLED(CONFIG_LOCKDEP)) {			\
> +			rcu_read_lock_sched_notrace();			\
> +			rcu_dereference_sched(__tracepoint_##name.funcs);\
> +			rcu_read_unlock_sched_notrace();		\
> +		}							\
>  	}								\
>  	__DECLARE_TRACE_RCU(name, PARAMS(proto), PARAMS(args),		\
>  		PARAMS(cond), PARAMS(data_proto), PARAMS(data_args))	\
> _
>