[PATCH -tip] Re: [PATCH -tip 0/2] Temporary RCU fixes for notrace and hotplug CPU

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

On Tue, Aug 25, 2009 at 02:02:33PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > On Tue, Aug 25, 2009 at 12:25:49PM -0400, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > > On Tue, Aug 25, 2009 at 10:00:47AM +0200, Ingo Molnar wrote:
> > > > > 
> > > > > btw., i'm still seeing crashes with the latest RCU bits:
> > > > > 
> > > > > [   20.621740] Testing event sys_enter_futex: OK
> > > > > [   20.629738] Testing event sys_exit_futex: OK
> > > > > [   20.637737] Testing event lock_acquire: [reboot]
> > > > > 
> > > > > Possibly due to infinite recursion as well. Config attached.
> > > > 
> > > > Color me confused...
> > > > 
> > > > Unless someone has a better idea, I will send in a patch that adds
> > > > "notrace" to every RCU API member used by any file in the kernel
> > > > that has "trace" in its name (excluding ptrace.c and rcutree_trace.c,
> > > > of course).  This list is as follows:
> > > > 
> > > > 	call_rcu()
> > > > 	call_rcu_sched()
> > > > 	rcu_read_lock()
> > > > 	rcu_read_unlock()
> > > > 
> > > > So, any better ideas?
> > > 
> > > Tracers using RCU should use the _notrace() version of read_lock/unlock.
> > > I think the callers should be fixed rather than RCU.
> > > 
> > > Tracepoints have been designed to use the _notrace variant on the
> > > instrumentation site. The core of tracepoint management use
> > > call_rcu_sched(), which can be traced without any problem.
> > > 
> > > I have not followed the late tracing development as closely though, so
> > > errors might have crept in.
> > 
> > Or I might have inadvertently broken something in a non-obvious (to me,
> > anyway) manner.
> > 
> > So, would you be willing to look at commit bc33f24bd in the -tip
> > tree and see if there is anything I broke other than the now-fixed
> > rcu_read_lock_sched_notrace() and rcu_read_unlock_sched_notrace()?
> > And for that matter, whether my alleged fix for these two API members
> > really does fix the problem (-tip commit 7c614d6461)?
> > 
> > The -tip tree is at:
> > 
> > 	git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git
> > 
> > And these patches are on the tip/core/rcu branch.
> > 
> 
> sure, here we go:
> 
>  static inline void rcu_read_lock_sched_notrace(void)
>  {
>         preempt_disable_notrace();
> +       __acquire(RCU_SCHED);
> +       rcu_read_acquire();
>  }
> 
> and
> 
> 
>  static inline void rcu_read_unlock_sched_notrace(void)
>  {
> +       rcu_read_release();
> +       __release(RCU_SCHED);
>         preempt_enable_notrace();
>  }
> 
> will make those _notrace primitives call into lockdep. I don't think
> this is correct, and this might be causing your problem.
> 
> rcu_read_acquire/release are calling lock_acquire/release, those should
> be removed.
> 
> __acquire() simply seems to be defined to a gcc "context" attribute,
> probably for the sparse checker. I think it should be safe to leave them
> there.

Thank you for looking this over, Mathieu!!!  Please see below for patch.

							Thanx, Paul

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

Remove lockdep annotations from RCU's _notrace() API members.

The lockdep annotations rcu_read_acquire() and rcu_read_release()
might lead to infinite looping if called from lockdep.  So this patch
removes them.

Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

 rcupdate.h |    2 --
 1 file changed, 2 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 8b4422c..95e0615 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -195,7 +195,6 @@ static inline notrace void rcu_read_lock_sched_notrace(void)
 {
 	preempt_disable_notrace();
 	__acquire(RCU_SCHED);
-	rcu_read_acquire();
 }
 
 /*
@@ -211,7 +210,6 @@ static inline void rcu_read_unlock_sched(void)
 }
 static inline notrace void rcu_read_unlock_sched_notrace(void)
 {
-	rcu_read_release();
 	__release(RCU_SCHED);
 	preempt_enable_notrace();
 }