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Re: [Qemu-devel] [RFC PATCH V2] qemu-char: Fix context for g_source_atta


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC PATCH V2] qemu-char: Fix context for g_source_attach()
Date: Fri, 8 Jul 2016 16:27:31 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1


On 08/07/2016 10:54, Daniel P. Berrange wrote:
> On Fri, Jul 08, 2016 at 09:48:23AM +0800, Fam Zheng wrote:
>> On Wed, 06/22 18:49, Zhang Chen wrote:
>>> We want to poll and handle chardev in another thread
>>> other than main loop. But qemu_chr_add_handlers() can only
>>> work for global default context other than thread default context.
>>> So we use g_source_attach(xx, g_main_context_get_thread_default())
>>> replace g_source_attach(xx, NULL) to attach g_source.
>>> Comments from jason.
>>>
>>> Signed-off-by: Zhang Chen <address@hidden>
>>> Signed-off-by: Jason Wang <address@hidden>
>>> ---
>>>  io/channel.c | 2 +-
>>>  qemu-char.c  | 6 +++---
>>>  2 files changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/io/channel.c b/io/channel.c
>>> index 692eb17..cd25677 100644
>>> --- a/io/channel.c
>>> +++ b/io/channel.c
>>> @@ -146,7 +146,7 @@ guint qio_channel_add_watch(QIOChannel *ioc,
>>>  
>>>      g_source_set_callback(source, (GSourceFunc)func, user_data, notify);
>>>  
>>> -    id = g_source_attach(source, NULL);
>>> +    id = g_source_attach(source, g_main_context_get_thread_default());
>>>      g_source_unref(source);
>>>  
>>>      return id;
>>> diff --git a/qemu-char.c b/qemu-char.c
>>> index 84f49ac..4340457 100644
>>> --- a/qemu-char.c
>>> +++ b/qemu-char.c
>>> @@ -859,7 +859,7 @@ static gboolean io_watch_poll_prepare(GSource *source, 
>>> gint *timeout_)
>>>          iwp->src = qio_channel_create_watch(
>>>              iwp->ioc, G_IO_IN | G_IO_ERR | G_IO_HUP | G_IO_NVAL);
>>>          g_source_set_callback(iwp->src, iwp->fd_read, iwp->opaque, NULL);
>>> -        g_source_attach(iwp->src, NULL);
>>> +        g_source_attach(iwp->src, g_main_context_get_thread_default());
>>>      } else {
>>>          g_source_destroy(iwp->src);
>>>          g_source_unref(iwp->src);
>>> @@ -918,7 +918,7 @@ static guint io_add_watch_poll(QIOChannel *ioc,
>>>      iwp->fd_read = (GSourceFunc) fd_read;
>>>      iwp->src = NULL;
>>>  
>>> -    tag = g_source_attach(&iwp->parent, NULL);
>>> +    tag = g_source_attach(&iwp->parent, 
>>> g_main_context_get_thread_default());
>>>      g_source_unref(&iwp->parent);
>>>      return tag;
>>>  }
>>> @@ -3982,7 +3982,7 @@ int qemu_chr_fe_add_watch(CharDriverState *s, 
>>> GIOCondition cond,
>>>      }
>>>  
>>>      g_source_set_callback(src, (GSourceFunc)func, user_data, NULL);
>>> -    tag = g_source_attach(src, NULL);
>>> +    tag = g_source_attach(src, g_main_context_get_thread_default());
>>>      g_source_unref(src);
>>>  
>>>      return tag;
>>> -- 
>>
>> IIRC this opens a gate for your special thread (COLO compare thread?) to use
>> QIOChannel.
> 
> I've no real objection to this proposed patch, though it is fairly pointless
> to take it now without seeing any following patch that actually makes use
> of this added feature.

I agree.

>> I think in the long run it is better to think about allowing integrating QIO 
>> to
>> AioContext, to support its usage outside main loop.  Given how opaque GSource
>> is, I'm not sure how feasible that is, or how useful it will be.  Anyway we
>> should definitely hear more opinions from Daniel and Paolo.
> 
> Personally I think it is preferable to stick as close to the standard GSource
> model as possible, as that's widely used & well understood API, compared to 
> the
> QEMU specific AioContext.

AioContext is more optimized for the case where the callbacks are
static.  In general this is not the case for qemu-char.c.

Paolo



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