netdev - Re: [PATCH net] tcp: make sure EPOLLOUT wont be missed

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Date:   Sat, 17 Aug 2019 13:10:44 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jason Baron <jbaron@...mai.com>,
        Vladimir Rutsky <rutsky@...gle.com>
Subject: Re: [PATCH net] tcp: make sure EPOLLOUT wont be missed

On Sat, Aug 17, 2019 at 12:26 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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>
> ---
>  net/core/stream.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks, Eric!

neal