From ce5ec440994b7b2b714633b516b9e0a4f6a98dfe Mon Sep 17 00:00:00 2001 From: Jason Baron Date: Wed, 20 May 2015 15:52:53 +0000 Subject: 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 Acked-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index ca1d476c80ef..0a3f9a00565b 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -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; } -- cgit v1.2.3