tcp: make sure EPOLLOUT wont be missed

As Jason Baron explained in commit 790ba456 ("tcp: set SOCK_NOSPACE
under memory pressure"), it is crucial we properly set SOCK_NOSPACE
when needed.

However, Jason patch had a bug, because the 'nonblocking' status
as far as sk_stream_wait_memory() is concerned is governed
by MSG_DONTWAIT flag passed at sendmsg() time :

    long timeo = sock_sndtimeo(sk, flags & MSG_DONTWAIT);

So it is very possible that tcp sendmsg() calls sk_stream_wait_memory(),
and that sk_stream_wait_memory() returns -EAGAIN with SOCK_NOSPACE
cleared, if sk->sk_sndtimeo has been set to a small (but not zero)
value.

This patch removes the 'noblock' variable since we must always
set SOCK_NOSPACE if -EAGAIN is returned.

It also renames the do_nonblock label since we might reach this
code path even if we were in blocking mode.

Fixes: 790ba456 ("tcp: set SOCK_NOSPACE under memory pressure")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jason Baron <jbaron@akamai.com>
Reported-by: Vladimir Rutsky  <rutsky@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/stream.c

@@ -120,7 +120,6 @@ int sk_stream_wait_memory(struct sock *sk, long *timeo_p)
 	int err = 0;
 	long vm_wait = 0;
 	long current_timeo = *timeo_p;
-	bool noblock = (*timeo_p ? false : true);
 	DEFINE_WAIT_FUNC(wait, woken_wake_function);
 
 	if (sk_stream_memory_free(sk))
@@ -133,11 +132,8 @@ int sk_stream_wait_memory(struct sock *sk, long *timeo_p)
 
 		if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))
 			goto do_error;
-		if (!*timeo_p) {
-			if (noblock)
-				set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
-			goto do_nonblock;
-		}
+		if (!*timeo_p)
+			goto do_eagain;
 		if (signal_pending(current))
 			goto do_interrupted;
 		sk_clear_bit(SOCKWQ_ASYNC_NOSPACE, sk);
@@ -169,7 +165,13 @@ int sk_stream_wait_memory(struct sock *sk, long *timeo_p)
 do_error:
 	err = -EPIPE;
 	goto out;
-do_nonblock:
+do_eagain:
+	/* Make sure that whenever EAGAIN is returned, EPOLLOUT event can
+	 * be generated later.
+	 * When TCP receives ACK packets that make room, tcp_check_space()
+	 * only calls tcp_new_space() if SOCK_NOSPACE is set.
+	 */
+	set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 	err = -EAGAIN;
 	goto out;
 do_interrupted: