Re: [Qemu-devel] [PATCH 03/18] qemu-thread: introduce QemuLockCnt

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/09/2015 10:49, Fam Zheng wrote:
>> > +    qemu_lockcnt_inc(&io_handlers_lockcnt);
>> > +    QLIST_FOREACH_RCU(ioh, &io_handlers, pioh) {
>> > +        if (ioh->revents & G_IO_OUT) {
>> > +            ioh->fd_write(ioh->opaque);
>> > +        }
>> > +    }
> I'm confused, the comment of QLIST_FOREACH_RCU says "list traversal
> must occur within an RCU critical section.", but there is not rcu_read_lock
> here. Why?

Right, the comment should be updated.

RCU can be seen as a "global reference count" that prevents freeing an
object between rcu_read_lock and rcu_read_unlock.  Here the reference
count is provided by the LockCnt.

The difference between QLIST_FOREACH and QLIST_FOREACH_RCU is just that
the latter has an extra smp_read_barrier_depends.  The barrier is needed
for all lockless visits.  I think QLIST_FOREACH_RCU is more expressive
than QLIST_FOREACH_LOCKLESS or something like that.

Paolo