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Re: [Qemu-block] [PATCH 03/11] util: use RCU accessors for notifiers

From: Paolo Bonzini
Subject: Re: [Qemu-block] [PATCH 03/11] util: use RCU accessors for notifiers
Date: Mon, 10 Jul 2017 18:06:32 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0

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).


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