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From: John Baldwin <jhb@freebsd.org>
To: freebsd-hackers@freebsd.org
Cc: Julian Kornberger <jk@kornberger.name>
Subject: Re: Usage of sosend() and its context
Date: Thu, 16 Jun 2016 09:40:03 -0700
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On Monday, June 13, 2016 11:08:31 PM Julian Kornberger wrote:
> Hi,
> 
> I am building a kernel module for fastd [1] and have the problem, that 
> on one of three machines the sosend() call seems to block. I use 
> sosend() [2] in a tunneling function that is set by 
> udp_set_kernel_tunneling().
> 
> Since I don't know which thread to pass to sosend() I just use 
> curthread. Is it possible that I should not use sosend() in this context 
> and instead build the IP packet manually and pass it to ip_output()? Any 
> other hints?
> 
> [1] https://fastd.readthedocs.io/
> [2] 
> https://github.com/digineo/fastd/blob/97fd40b9992778a3bf11756f41aa7c2615bba2f3/kmod/fastd.c#L664

It's possible to ask sosend() to not block and fail with EWOULDBLOCK instead
by passing MSG_NOWAIT in flags.  However, you might still block on sblock(), so
you may need to defer your work to a sleepable context using something like a
taskqueue.

-- 
John Baldwin