Re: [PATCH 6/5]rcu/tracing: Add rcu_disabled to denote when rcu_irq_enter() will not work

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

On Fri, Apr 07, 2017 at 12:35:16PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> Tracing uses rcu_irq_enter() as a way to make sure that RCU is watching when
> it needs to use rcu_read_lock() and friends. This is because tracing can
> happen as RCU is about to enter user space, or about to go idle, and RCU
> does not watch for RCU read side critical sections as it makes the
> transition.
> 
> There is a small location within the RCU infrastructure that rcu_irq_enter()
> itself will not work. If tracing were to occur in that section it will break
> if it tries to use rcu_irq_enter().
> 
> Originally, this happens with the stack_tracer, because it will call
> save_stack_trace when it encounters stack usage that is greater than any
> stack usage it had encountered previously. There was a case where that
> happened in the RCU section where rcu_irq_enter() did not work, and lockdep
> complained loudly about it. To fix it, stack tracing added a call to be
> disabled and RCU would disable stack tracing during the critical section
> that rcu_irq_enter() was inoperable. This solution worked, but there are
> other cases that use rcu_irq_enter() and it would be a good idea to let RCU
> give a way to let others know that rcu_irq_enter() will not work. For
> example, in trace events.
> 
> Another helpful aspect of this change is that it also moves the per cpu
> variable called in the RCU critical section into a cache locale along with
> other RCU per cpu variables used in that same location.
> 
> I'm keeping the stack_trace_disable() code, as that still could be used in
> the future by places that really need to disable it. And since it's only a
> static inline, it wont take up any kernel text if it is not used.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Looks better, especially __this_cpu_inc() and __this_cpu_dec().

How about rcu_irq_enter_disabled instead of rcu_disabled?  We aren't
really disabling all of RCU.  ;-)

							Thanx, Paul

> ---
>  include/linux/rcupdate.h   |  5 +++++
>  kernel/rcu/tree.c          | 18 ++++++++++++++++--
>  kernel/trace/trace_stack.c |  8 ++++++++
>  3 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index de88b33..d922ee7 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -97,6 +97,7 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
>  			       unsigned long secs,
>  			       unsigned long c_old,
>  			       unsigned long c);
> +bool rcu_is_disabled(void);
>  #else
>  static inline void rcutorture_get_gp_data(enum rcutorture_type test_type,
>  					  int *flags,
> @@ -113,6 +114,10 @@ static inline void rcutorture_record_test_transition(void)
>  static inline void rcutorture_record_progress(unsigned long vernum)
>  {
>  }
> +bool rcu_is_disabled(void)
> +{
> +	return false;
> +}
>  #ifdef CONFIG_RCU_TRACE
>  void do_trace_rcu_torture_read(const char *rcutorturename,
>  			       struct rcu_head *rhp,
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8b4d273..727325f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -285,6 +285,20 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
>  };
> 
>  /*
> + * There's a few places, currently just in the tracing infrastructure,
> + * that uses rcu_irq_enter() to make sure RCU is watching. But there's
> + * a small location where that will not even work. In those cases
> + * rcu_is_disabled() needs to be checked to make sure rcu_irq_enter()
> + * can be called.
> + */
> +static DEFINE_PER_CPU(bool, rcu_disabled);
> +
> +bool rcu_is_disabled(void)
> +{
> +	return this_cpu_read(rcu_disabled);
> +}
> +
> +/*
>   * Record entry into an extended quiescent state.  This is only to be
>   * called when not already in an extended quiescent state.
>   */
> @@ -800,10 +814,10 @@ static void rcu_eqs_enter_common(bool user)
>  		do_nocb_deferred_wakeup(rdp);
>  	}
>  	rcu_prepare_for_idle();
> -	stack_tracer_disable();
> +	__this_cpu_inc(rcu_disabled);
>  	rdtp->dynticks_nesting = 0; /* Breaks tracing momentarily. */
>  	rcu_dynticks_eqs_enter(); /* After this, tracing works again. */
> -	stack_tracer_enable();
> +	__this_cpu_dec(rcu_disabled);
>  	rcu_dynticks_task_enter();
> 
>  	/*
> diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> index f2f02ff..aec2df1 100644
> --- a/kernel/trace/trace_stack.c
> +++ b/kernel/trace/trace_stack.c
> @@ -96,6 +96,14 @@ check_stack(unsigned long ip, unsigned long *stack)
>  	if (in_nmi())
>  		return;
> 
> +	/*
> +	 * There's a slight chance that we are tracing inside the
> +	 * RCU infrastructure, and rcu_irq_enter() will not work
> +	 * as expected.
> +	 */
> +	if (unlikely(rcu_is_disabled()))
> +		return;
> +
>  	local_irq_save(flags);
>  	arch_spin_lock(&stack_trace_max_lock);
> 
> -- 
> 2.9.3
>