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

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Message-ID: <1432141796.4060.28.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 20 May 2015 10:09:56 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Baron <jbaron@...mai.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2] tcp: ensure epoll edge trigger wakeup when
 write queue is empty

On Wed, 2015-05-20 at 15:52 +0000, Jason Baron wrote:
> 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>
> ---

Acked-by: Eric Dumazet <edumazet@...gle.com>


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