[systemd-devel] [PATCH kdbus] handle: Return POLLOUT | POLLWRNORM mask when no messages are pending

Marcel Holtmann marcel at holtmann.org
Fri Aug 15 14:16:50 PDT 2014

Hi Daniel,

>> To facility the feature of doing an asynchronous sending of messages
>> when the bus is idle, make sure to return POLLOUT | POLLWRNORM from
>> kdbus_handle_poll.
>> Signed-off-by: Marcel Holtmann <marcel at holtmann.org>
>> ---
>> handle.c | 2 ++
>> 1 file changed, 2 insertions(+)
>> diff --git a/handle.c b/handle.c
>> index ac6868133280..fc15d28351b3 100644
>> --- a/handle.c
>> +++ b/handle.c
>> @@ -884,6 +884,8 @@ static unsigned int kdbus_handle_poll(struct file *file,
>> 		mask |= POLLERR | POLLHUP;
>> 	else if (!list_empty(&conn->msg_list))
>> 		mask |= POLLIN | POLLRDNORM;
>> +	else
>> +		mask |= POLLOUT | POLLWRNORM;
> Hmm, what's your use case here? list_empty(&conn->msg_list) only checks
> the incoming list of the current connection for pending messages. That
> doesn't tell you whether the bus is idle, or if your receiving end has
> messages pending ...

if you are a good client citizen, then you only send messages when POLLOUT gets signaled. That is what this is allowing now.

Blindly sending messages is never a good idea. You want to poll for POLLOUT first. This does not make a big difference for current kernel, but it allows future extensions when the clients are well behaving and just waiting for POLLOUT. Meaning once the kernel does not signal POLLOUT, no new messages will come from the client.

Current code does not do this POLLOUT before sending a messages, but our kdbus client actually does that. It is the right thing to do. And we have been doing this with all of our protocols that are using asynchronous IO. No point in kdbus being any different.



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