Re: __update_max_tr: rcu_read_lock() used illegally while idle!

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

On Tue, Jul 31, 2012 at 11:45:02AM -0400, Steven Rostedt wrote:
> On Tue, 2012-07-31 at 08:18 -0700, Paul E. McKenney wrote:
> > On Tue, Jul 31, 2012 at 10:56:23AM -0400, Steven Rostedt wrote:
> > > On Tue, 2012-07-31 at 10:51 -0400, Steven Rostedt wrote:
> > > 
> > > > > OK, I will bite.  How about using something like RCU_NONIDLE(), either
> > > > > directly or open-coded, to make it a legal call site?
> > > > 
> > > > OK, then something like:
> > > > 
> > > > 	RCU_NONIDLE(max_data = task_uid(tsk));
> > > > 
> > > > would work when called normally or with idle?
> > > > 
> > > 
> > > The comment above RCU_NONIDLE() says:
> > > 
> > > 	This macro may be used from process-level code only.
> > > 
> > > Although I'm not sure what a 'level' is. Do you mean process-context? If
> > > so, then this will not work because it can be called from non process
> > > level code (return from interrupt), or any interrupt that enables
> > > interrupts.
> > 
> > Yep, process context.  It seems that I was naively expecting the
> > calls in interrupt context to be covered by rcu_irq_enter() and
> > rcu_irq_exit().  I take it that these calls are happening outside of
> > the rcu_irq_enter()-protected region?
> 
> Yep, it is called by the same hooks that lockdep uses. TRACE_IRQS_ON
> inside the irq handler in assembly.

Does the following patch help?

							Thanx, Paul

------------------------------------------------------------------------

rcu: Permit RCU_NONIDLE() to be used from interrupt context

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.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 115ead2..4eab81a 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_exit(); \
 		do { a; } while (0); \
-		rcu_idle_enter(); \
+		rcu_irq_enter(); \
 	} while (0)
 
 /*
diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 31a10f9..aee6df8 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 44609c3..2175b13 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -455,6 +455,7 @@ 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
@@ -550,6 +551,7 @@ 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