DragonFly kernel List (threaded) for 2008-08
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DragonFly kernel List (threaded) for 2008-08
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Re: [netmp] socket accesses

From: Aggelos Economopoulos <aoiko@xxxxxxxxxxxxxx>
Date: Mon, 18 Aug 2008 01:18:53 +0300

On Saturday 09 August 2008, Matthew Dillon wrote:
>     Generally speaking what I am recommending is that races be allowed
>     insofar as they don't interfere with operation, which will be in
>     most cases.  The cases where it would interfere are primarily going
>     to be related to userland blocking conditions (e.g. read() on socket
>     blocks waiting for incoming data/disconnect/shutdown, or write() on
>     socket blocks waiting for buffer space to become available).
>     These blocking conditions can be resolved by sending a synchronous
>     message to the protocol stack, just like we do now for connect() and
>     numerous other operations.  The protocol stack would then be able
>     to resolve any races that occured, potentially returning the message
>     (unblocking the connection) immediately in the rare case of a race.
> 					---
>     Ok, here are my suggestions:
> 	* These should only be set or cleared by the protocol stack,
> 	  except for a degenerate case in unp_connect2() (for socketpair()
> 	  I think).
> 	* While it is true that once one is set, then at least one or the
> 	  other will be found to be set under normal operation.  But
> 	  remote disconnection will still race, and that's an issue.
> 	* I believe that all such races can be resolved in the ssb_wait()
> 	  code.  Instead of setting a flag and tsleep()ing this code could
> 	  message the protocol stack (the same way we do for e.g. connect()).
> 	  The protocol stack would then resolve the races, potentially
> 	  returning the message immediately if .e.g. both bits were found
> 	  to be clear.
> 	* Set by both sides.  Should be ok as long as any races are resolved.
> 	  In the case of userland, it could set CANTRCVMORE and send an
> 	  async message to the protocol stack.
> 	  In the case of the protocol stack, it could set CANTRCVMORE and
> 	  then drain any wait-style messages (such as sent by soreceive()
> 	  when it blocks via ssb_wait(), using the new idea for ssb_wait()).

What do you picture the protocol side doing in response to this message?
Currently, the process side will be woken up when e.g. new data arrives or
the connection state changes etc. But to decide if anything changed since
the message was sent, the protocol essentially needs to know what the process
side "knew" when it sent the message. So we need to send a message which
includes a pointer to, say:

struct socksnap {
	int state;
	int rcvdata;

Is that what you had in mind?


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