Re: [RFC Patch 1/10] Introducing generic hardware breakpoint handlerinterfaces

[Alan Stern]

On Sun, 1 Feb 2009, Paul E. McKenney wrote:

> > > Yes, indeed. With the current implementation, there's a possibility of
> > > two instances of update_this_cpu() function executing - one with an
> > > rcu_read_lock() taken (when called from load_debug_registers) while the
> > > other without (when invoked through update_all_cpus()).
> > 
> > No, this isn't possible unless I have misunderstood the nature of
> > IPIs.  Isn't is true that calling local_irq_save() will block delivery
> > of IPIs?
> 
> Touche!  ;-)
> 
> But in that case, why do you need the synchronize_rcu() following the
> on_each_cpu() above?  Is this needed to make sure that any concurrent
> load_debug_registers() call has completed?

No; it's needed to make sure that any concurrent
switch_to_thread_hw_breakpoint() call has completed.  That's where the
important RCU read lock is taken.  The routine is called not just by
update_this_cpu() (and indirectly by load_debug_registers()) but also
by __register_user_hw_breakpoint(), __unregister_user_hw_breakpoint(),
and the task-switch routine.

It's possible that the IPI from on_each_cpu() could interrupt an
instance of switch_to_thread_hw_breakpoint() -- thereby causing it to
run recursively.  After the inner instance returns and the IPI is over,
the outer instance will realize what has happened and restart itself.  
synchronize_rcu() insures that update_all_cpus() will wait until the
outer instance is done.

In fact, the RCU read lock in load_debug_registers() probably isn't 
necessary.  But it's cleaner to leave it in; it points out that the 
routine accesses data structures which are protected by RCU.

Alan Stern

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