Commit ce5ec440 authored May 20, 2015 by Jason Baron Committed by David S. Miller May 21, 2015

tcp: ensure epoll edge trigger wakeup when write queue is empty



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@akamai.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent b92d5814

net/ipv4/tcp.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -999,6 +999,9 @@ do_error:
		if (copied)
		goto out;
		out_err:
		/* make sure we wake any epoll edge trigger waiter */
		if (unlikely(skb_queue_len(&sk->sk_write_queue) == 0 && err == -EAGAIN))
		sk->sk_write_space(sk);
		return sk_stream_error(sk, flags, err);
		}

		@@ -1288,6 +1291,9 @@ do_error:
		goto out;
		out_err:
		err = sk_stream_error(sk, flags, err);
		/* make sure we wake any epoll edge trigger waiter */
		if (unlikely(skb_queue_len(&sk->sk_write_queue) == 0 && err == -EAGAIN))
		sk->sk_write_space(sk);
		release_sock(sk);
		return err;
		}

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