Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section?

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

On Thu, Sep 05, 2013 at 01:59:59PM -0700, Paul E. McKenney wrote:
> On Thu, Sep 05, 2013 at 04:25:58PM -0400, Steven Rostedt wrote:
> > On Thu, 5 Sep 2013 12:52:34 -0700
> > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > 
> > > There is currently no way for kernel code to determine whether it
> > > is safe to enter an RCU read-side critical section, in other words,
> > > whether or not RCU is paying attention to the currently running CPU.
> > > Given the large and increasing quantity of code shared by the idle loop
> > > and non-idle code, the this shortcoming is becoming increasingly painful.
> > > 
> > > This commit therefore adds rcu_watching_this_cpu(), which returns true
> > > if it is safe to enter an RCU read-side critical section on the currently
> > > running CPU.  This function is quite fast, using only a __this_cpu_read().
> > > However, the caller must disable preemption.
> > > 
> > > Reported-by: Steven Rostedt <rostedt@goodmis.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> > Thanks Paul!
> > 
> > > 
> > >  include/linux/rcupdate.h |    1 +
> > >  kernel/rcutree.c         |   12 ++++++++++++
> > >  2 files changed, 13 insertions(+)
> > > 
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index 15d33d9..1c7112c 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -225,6 +225,7 @@ extern void rcu_idle_enter(void);
> > >  extern void rcu_idle_exit(void);
> > >  extern void rcu_irq_enter(void);
> > >  extern void rcu_irq_exit(void);
> > > +extern bool rcu_watching_this_cpu(void);
> > >  
> > >  #ifdef CONFIG_RCU_USER_QS
> > >  extern void rcu_user_enter(void);
> > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > > index a06d172..7b8fcee 100644
> > > --- a/kernel/rcutree.c
> > > +++ b/kernel/rcutree.c
> > 
> > Don't we also need a rcutiny version too? Otherwise I'm guessing that
> > it wont compile if I use it with rcutiny (even if it always returns
> > true).
> 
> Yep, there needs to be an rcutiny version, and it must work in the
> same way.  So what I currently have is that the rcutiny version is under
> CONFIG_RCU_TRACE.  This will require some Kconfig monkeying somewhere
> to select CONFIG_RCU_TRACE if CONFIG_SMP and tracing is enabled.
> 
> Just trying to keep rcutiny tiny...

And here is the updated patch, for whatever it is worth.

							Thanx, Paul
------------------------------------------------------------------------

rcu: Is it safe to enter an RCU read-side critical section?

There is currently no way for kernel code to determine whether it
is safe to enter an RCU read-side critical section, in other words,
whether or not RCU is paying attention to the currently running CPU.
Given the large and increasing quantity of code shared by the idle loop
and non-idle code, the this shortcoming is becoming increasingly painful.

This commit therefore adds rcu_watching_this_cpu(), which returns true
if it is safe to enter an RCU read-side critical section on the currently
running CPU.  This function is quite fast, using only a __this_cpu_read().
However, the caller must disable preemption.

Reported-by: Steven Rostedt <rostedt@goodmis.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 15d33d9..7c024fd 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -225,6 +225,9 @@ extern void rcu_idle_enter(void);
 extern void rcu_idle_exit(void);
 extern void rcu_irq_enter(void);
 extern void rcu_irq_exit(void);
+#if defined(CONFIG_SMP) || defined(CONFIG_RCU_TRACE)
+extern bool rcu_watching_this_cpu(void);
+#endif /* #if defined(CONFIG_SMP) || defined(CONFIG_RCU_TRACE) */
 
 #ifdef CONFIG_RCU_USER_QS
 extern void rcu_user_enter(void);
diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 7e3b0d6..fce820f 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -189,6 +189,17 @@ EXPORT_SYMBOL(rcu_is_cpu_idle);
 
 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
+#ifdef CONFIG_RCU_TRACE
+/*
+ * Test whether the current CPU can safely enter RCU read-side critical
+ * sections.  The caller must at least have disabled interrupts.
+ */
+bool rcu_watching_this_cpu(void)
+{
+	return !!rcu_dynticks_nesting;
+}
+#endif /* #ifdef CONFIG_RCU_TRACE */
+
 /*
  * Test whether the current CPU was interrupted from idle.  Nested
  * interrupts don't count, we must be running at the first interrupt
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a06d172..7b8fcee 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -710,6 +710,18 @@ EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
 #endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
 
 /**
+ * rcu_watching_this_cpu - are RCU read-side critical sections safe?
+ *
+ * Return true if RCU is watching the running CPU, which means that this
+ * CPU can safely enter RCU read-side critical sections.  The caller must
+ * have at least disabled preemption.
+ */
+bool rcu_watching_this_cpu(void)
+{
+	return !!__this_cpu_read(rcu_dynticks.dynticks_nesting);
+}
+
+/**
  * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle
  *
  * If the current CPU is idle or running at a first-level (not nested)