netdev - Re: [PATCH net-next v2] tcp: ensure epoll edge trigger wakeup when write queue is empty

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Message-Id: <20150521.185301.1643144249667063842.davem@davemloft.net>
Date:	Thu, 21 May 2015 18:53:01 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	jbaron@...mai.com
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2] tcp: ensure epoll edge trigger wakeup when
 write queue is empty

From: Jason Baron <jbaron@...mai.com>
Date: Wed, 20 May 2015 15:52:53 +0000 (GMT)

> From: Jason Baron <jbaron@...mai.com>
> 
> We currently rely on the setting of SOCK_NOSPACE in the write()
> path to ensure that we wake up any epoll edge trigger waiters when
> acks return to free space in the write queue. However, if we fail
> to allocate even a single skb in the write queue, we could end up
> waiting indefinitely.
> 
> Fix this by explicitly issuing a wakeup when we detect the condition
> of an empty write queue and a return value of -EAGAIN. This allows
> userspace to re-try as we expect this to be a temporary failure.
> 
> I've tested this approach by artificially making
> sk_stream_alloc_skb() return NULL periodically. In that case,
> epoll edge trigger waiters will hang indefinitely in epoll_wait()
> without this patch.
> 
> Signed-off-by: Jason Baron <jbaron@...mai.com>
> ---
> v2:
> - ensure it compiles :)

Applied, thanks Jason :)
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