Re: [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be used from interrupt context

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

On Fri, Aug 31, 2012 at 11:00:52AM -0700, Josh Triplett wrote:
> On Thu, Aug 30, 2012 at 11:56:17AM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> > 
> > There is a need to use RCU from interrupt context, but either before
> > rcu_irq_enter() is called or after rcu_irq_exit() is called.  If the
> > interrupt occurs from idle, then lockdep-RCU will complain about such
> > uses, as they appear to be illegal uses of RCU from the idle loop.
> > In other environments, RCU_NONIDLE() could be used to properly protect
> > the use of RCU, but RCU_NONIDLE() currently cannot be invoked except
> > from process context.
> > 
> > This commit therefore modifies RCU_NONIDLE() to permit its use more
> > globally.
> > 
> > Reported-by: Steven Rostedt <rostedt@goodmis.org>
> > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Something seems wrong about this.  The addition of EXPORT_SYMBOL_GPL
> suggests that such interrupt handlers might live in modules.  In what
> situation might a module interrupt handler get called from the idle
> loop, before rcu_irq_enter or after rcu_irq_exit, and need to know that
> when using RCU?

Drivers can be in modules, in which case their interrupt handlers will
also be in the corresponding module.  I do agree that in most cases,
the irq_enter() and irq_exit() hooks would be invoked by non-module code,
but I do believe that I had to add those exports due to build failures.

Steven will let me know if I am confused on this point.

							Thanx, Paul

> - Josh Triplett
> 
> > ---
> >  include/linux/rcupdate.h |    6 ++----
> >  kernel/rcutiny.c         |    2 ++
> >  kernel/rcutree.c         |    2 ++
> >  3 files changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 115ead2..0fbbd52 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -210,14 +210,12 @@ extern void exit_rcu(void);
> >   * to nest RCU_NONIDLE() wrappers, but the nesting level is currently
> >   * quite limited.  If deeper nesting is required, it will be necessary
> >   * to adjust DYNTICK_TASK_NESTING_VALUE accordingly.
> > - *
> > - * This macro may be used from process-level code only.
> >   */
> >  #define RCU_NONIDLE(a) \
> >  	do { \
> > -		rcu_idle_exit(); \
> > +		rcu_irq_enter(); \
> >  		do { a; } while (0); \
> > -		rcu_idle_enter(); \
> > +		rcu_irq_exit(); \
> >  	} while (0)
> >  
> >  /*
> > diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> > index e4163c5..2e073a2 100644
> > --- a/kernel/rcutiny.c
> > +++ b/kernel/rcutiny.c
> > @@ -115,6 +115,7 @@ void rcu_irq_exit(void)
> >  	rcu_idle_enter_common(newval);
> >  	local_irq_restore(flags);
> >  }
> > +EXPORT_SYMBOL_GPL(rcu_irq_exit);
> >  
> >  /* Common code for rcu_idle_exit() and rcu_irq_enter(), see kernel/rcutree.c. */
> >  static void rcu_idle_exit_common(long long oldval)
> > @@ -172,6 +173,7 @@ void rcu_irq_enter(void)
> >  	rcu_idle_exit_common(oldval);
> >  	local_irq_restore(flags);
> >  }
> > +EXPORT_SYMBOL_GPL(rcu_irq_enter);
> >  
> >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >  
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index f280e54..96b8aff 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -447,6 +447,7 @@ void rcu_irq_exit(void)
> >  		rcu_idle_enter_common(rdtp, oldval);
> >  	local_irq_restore(flags);
> >  }
> > +EXPORT_SYMBOL_GPL(rcu_irq_exit);
> >  
> >  /*
> >   * rcu_idle_exit_common - inform RCU that current CPU is moving away from idle
> > @@ -542,6 +543,7 @@ void rcu_irq_enter(void)
> >  		rcu_idle_exit_common(rdtp, oldval);
> >  	local_irq_restore(flags);
> >  }
> > +EXPORT_SYMBOL_GPL(rcu_irq_enter);
> >  
> >  /**
> >   * rcu_nmi_enter - inform RCU of entry to NMI context
> > -- 
> > 1.7.8
> > 
>