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From: Juan Quintela <quintela@redhat.com>
In-Reply-To: <20170720093455.GC23385@pxdev.xzpeter.org> (Peter Xu's message of
	"Thu, 20 Jul 2017 17:34:55 +0800")
References: <20170717134238.1966-1-quintela@redhat.com>
	<20170717134238.1966-10-quintela@redhat.com>
	<20170720093455.GC23385@pxdev.xzpeter.org>
Reply-To: quintela@redhat.com
Date: Tue, 08 Aug 2017 11:19:35 +0200
Message-ID: <874ltibemw.fsf@secure.mitica>
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Subject: Re: [Qemu-devel] [PATCH v5 09/17] migration: Start of multiple fd
 work
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To: Peter Xu <peterx@redhat.com>
Cc: qemu-devel@nongnu.org, dgilbert@redhat.com, lvivier@redhat.com, berrange@redhat.com

Peter Xu <peterx@redhat.com> wrote:
> On Mon, Jul 17, 2017 at 03:42:30PM +0200, Juan Quintela wrote:
>
> [...]
>
>>  int multifd_load_setup(void)
>>  {
>>      int thread_count;
>> -    uint8_t i;
>>  
>>      if (!migrate_use_multifd()) {
>>          return 0;
>>      }
>>      thread_count = migrate_multifd_threads();
>>      multifd_recv_state = g_malloc0(sizeof(*multifd_recv_state));
>> -    multifd_recv_state->params = g_new0(MultiFDRecvParams, thread_count);
>> +    multifd_recv_state->params = g_new0(MultiFDRecvParams *, thread_count);
>>      multifd_recv_state->count = 0;
>> -    for (i = 0; i < thread_count; i++) {
>> -        char thread_name[16];
>> -        MultiFDRecvParams *p = &multifd_recv_state->params[i];
>> -
>> -        qemu_mutex_init(&p->mutex);
>> -        qemu_sem_init(&p->sem, 0);
>> -        p->quit = false;
>> -        p->id = i;
>> -        snprintf(thread_name, sizeof(thread_name), "multifdrecv_%d", i);
>> -        qemu_thread_create(&p->thread, thread_name, multifd_recv_thread, p,
>> -                           QEMU_THREAD_JOINABLE);
>> -        multifd_recv_state->count++;
>> -    }
>
> Could I ask why we explicitly switched from MultiFDRecvParams[] array
> into a pointer array? Can we still use the old array?  Thanks,

Now, we could receive the channels out of order (the wonders of
networking).  So, we have two options that I can see:

* Add interesting global locking to be able to modify inplace (I know
  that it should be safe, but yet).
* Create a new struct in the new connection, and then atomically switch
  the pointer to the right instruction.

I can assure you that the second one makes it much more easier to detect
when you use the "channel" before you have fully created it O:-)

Later, Juan.