linux-kernel - [PATCH 5.2 054/143] tcp: make sure EPOLLOUT wont be missed

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Message-Id: <20190904175316.182242667@linuxfoundation.org>
Date:   Wed,  4 Sep 2019 19:53:17 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Jason Baron <jbaron@...mai.com>,
        Vladimir Rutsky <rutsky@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 5.2 054/143] tcp: make sure EPOLLOUT wont be missed

From: Eric Dumazet <edumazet@...gle.com>

[ Upstream commit ef8d8ccdc216f797e66cb4a1372f5c4c285ce1e4 ]

As Jason Baron explained in commit 790ba4566c1a ("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: 790ba4566c1a ("tcp: set SOCK_NOSPACE under memory pressure")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Jason Baron <jbaron@...mai.com>
Reported-by: Vladimir Rutsky  <rutsky@...gle.com>
Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
Acked-by: Neal Cardwell <ncardwell@...gle.com>
Acked-by: Jason Baron <jbaron@...mai.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/core/stream.c |   16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

--- a/net/core/stream.c
+++ b/net/core/stream.c
@@ -120,7 +120,6 @@ int sk_stream_wait_memory(struct sock *s
 	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 *s
 
 		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 @@ out:
 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: