Re: [Qemu-devel] [PATCH v3 5/6] migration: implement bi-directional RDMA QIOChannel

[858585 jemmy]

On Wed, May 16, 2018 at 5:36 PM, 858585 jemmy <address@hidden> wrote:
> On Tue, May 15, 2018 at 10:54 PM, Paolo Bonzini <address@hidden> wrote:
>> On 05/05/2018 16:35, Lidong Chen wrote:
>>> @@ -2635,12 +2637,20 @@ static ssize_t qio_channel_rdma_writev(QIOChannel 
>>> *ioc,
>>>  {
>>>      QIOChannelRDMA *rioc = QIO_CHANNEL_RDMA(ioc);
>>>      QEMUFile *f = rioc->file;
>>> -    RDMAContext *rdma = rioc->rdma;
>>> +    RDMAContext *rdma;
>>>      int ret;
>>>      ssize_t done = 0;
>>>      size_t i;
>>>      size_t len = 0;
>>>
>>> +    rcu_read_lock();
>>> +    rdma = atomic_rcu_read(&rioc->rdmaout);
>>> +
>>> +    if (!rdma) {
>>> +        rcu_read_unlock();
>>> +        return -EIO;
>>> +    }
>>> +
>>>      CHECK_ERROR_STATE();
>>>
>>>      /*
>>
>> I am not sure I understand this.  It would probably be wrong to use the
>> output side from two threads at the same time, so why not use two mutexes?
>
> Two thread will not invoke qio_channel_rdma_writev at the same time.
> The source qemu, migration thread only use writev, and the return path
> thread only
> use readv.
> The destination qemu already have a mutex mis->rp_mutex to make sure
> not use writev
> at the same time.
>
> The rcu_read_lock is used to protect not use RDMAContext when another
> thread closes it.

Any suggestion?

>
>>
>> Also, who is calling qio_channel_rdma_close in such a way that another
>> thread is still using it?  Would it be possible to synchronize with the
>> other thread *before*, for example with qemu_thread_join?
>
> The MigrationState structure includes to_dst_file and from_dst_file
> QEMUFile, the two QEMUFile use the same QIOChannel.
> For example, if the return path thread call
> qemu_fclose(ms->rp_state.from_dst_file),
> It will also close the RDMAContext for ms->to_dst_file.
>
> For live migration, the source qemu invokes qemu_fclose in different
> threads include main thread, migration thread, return path thread.
>
> The destination qemu invokes qemu_fclose in main thread, listen thread and
> COLO incoming thread.
>
> I do not find an effective way to synchronize these threads.
>
> Thanks.
>
>>
>> Thanks,
>>
>> Paolo