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

[Steven Rostedt]

On Fri, 7 Apr 2017 09:42:20 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> On Fri, Apr 07, 2017 at 12:35:16PM -0400, Steven Rostedt wrote:
> > From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxx>  
> 
> 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.  ;-)

OK, I'll make the update and send a 6.1 (and also a new 7/5)!

-- Steve