Re: [Qemu-devel] [PATCH 03/12] rcu: add rcu library

[Paolo Bonzini]

Il 17/05/2013 06:36, liu ping fan ha scritto:
>> > +    /* Mark the writer thread offline to make sure we don't wait for
>> > +     * our own quiescent state. This allows using synchronize_rcu()
>> > +     * in threads registered as readers.
>> > +     *
>> > +     * rcu_thread_offline() and rcu_thread_online() include a
>> > +     * memory barrier.
>> > +     */
>> > +    if (was_online) {
>> > +        rcu_thread_offline();
> Encourage the user to call synchronize_rcu() in reader?

Not in a read-side critical section, but in a *thread registered as
reader*.  And in QEMU, all threads actually are registered as readers.

> I think the caller should ensure it is outside read-section.

That would be possible by adding a small overhead to rcu_read_lock/unlock.

> Also online can be
> nested which make the situation even worse.

It's not online that can be nested, only offline.  So:

- if the thread is already marked as offline, there will be no effect.

- if the thread is not marked as offline, it will be.

Paolo