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

Ingo Molnar
2005-03-23T21:47:18Z

* 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