Re: [patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.41-07

[Ingo Molnar <mingo@elte.hu>]

* Ingo Molnar <mingo@elte.hu> wrote:

> i think the 'migrate read-count' method is not adequate either, 
> because all callbacks queued within an RCU read section must be called 
> after the lock has been dropped - while with the migration method 
> CPU#1 would be free to process callbacks queued in the RCU read 
> section still active on CPU#2.
> 
> i'm wondering how much of a problem this is though. Can there be stale 
> pointers at that point? Yes in theory, because code like:
> 
> 	rcu_read_lock();
> 	call_rcu(&dentry->d_rcu, d_callback);
> 	func(dentry->whatever);
> 	rcu_read_unlock();

but, this cannot happen, because call_rcu() is used by RCU-write code.

so the important property seems to be that any active RCU-read section 
should keep at least one CPU's active_readers count elevated 
permanently, for the duration of the RCU-read section. It doesnt matter 
that the reader migrates between CPUs - because the RCU code itself 
guarantees that no callbacks will be executed until _all_ CPUs have been 
in quiescent state. I.e. all CPUs have to go through a zero 
active_readers value before the callback is done.

	Ingo