[PATCH 4.9 58/83] tcp: make sure EPOLLOUT wont be missed

From: Greg Kroah-Hartman
Date: Wed Sep 04 2019 - 14:01:09 EST

From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ 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)

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@xxxxxxxxxx>
Cc: Jason Baron <jbaron@xxxxxxxxxx>
Reported-by: Vladimir Rutsky <rutsky@xxxxxxxxxx>
Acked-by: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>
Acked-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Acked-by: Jason Baron <jbaron@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
net/core/stream.c | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)

--- a/net/core/stream.c
+++ b/net/core/stream.c
@@ -118,7 +118,6 @@ 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);

if (sk_stream_memory_free(sk))
@@ -131,11 +130,8 @@ 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);
@@ -167,7 +163,13 @@ out:
err = -EPIPE;
goto out;
+ /* 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;