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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* 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.

Mathieu

> 							Thanx, Paul

-- 
Mathieu Desnoyers
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