On 10/07/2017 17:52, Stefan Hajnoczi wrote:
> On Thu, Jul 06, 2017 at 06:38:20PM +0200, Paolo Bonzini wrote:
>> void notifier_list_notify(NotifierList *list, void *data)
>> {
>> Notifier *notifier, *next;
>>
>> - QLIST_FOREACH_SAFE(notifier, &list->notifiers, node, next) {
>> + QLIST_FOREACH_SAFE_RCU(notifier, &list->notifiers, node, next) {
>> notifier->notify(notifier, data);
>> }
>> }
>
> Who calls rcu_read_lock() or is it unnecessary?It depends. If the notifier is really only used within the BQL, it's unnecessary. If the notifier's readers want to protect the notifier with RCU, it's up to the callers indeed. However, RCU accessors can also be used with any API that has the same contract as synchronize_rcu, i.e. it stops until all concurrent readers complete, no matter how "readers" are defined. In the next patch, for example, synchronize_rcu's role is taken by bdrv_drain (which is a superset of synchronize_rcu, since it also blocks new incoming readers). For a similar example in Linux, see drivers/vhost/net.c. It replaces rcu_read_lock/unlock with "always run readers for a workqueue", and synchronize_rcu with vhost_poll_flush (which calls vhost_work_flush). Paolo
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