linux-kernel - Strange issues with epoll since 5.0

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Message-ID: <CA+8F9hicnF=kvjXPZFQy=Pa2HJUS3JS+G9VswFHNQQynPMHGVQ@mail.gmail.com>
Date:   Mon, 15 Apr 2019 11:02:42 -0700
From:   Omar Kilani <omar.kilani@...il.com>
To:     linux-kernel@...r.kernel.org
Subject: Strange issues with epoll since 5.0

Hi there,

I’m still trying to piece together a reproducible test that triggers
this, but I wanted to post in case someone goes “hmmm... change X
might have done this”.

Basically, something’s broken (or at least, has changed enough to
cause problems in user space) in epoll since 5.0. It’s still broken in
5.1-rc5.

It doesn’t happen 100% of the time. It’s sort of hard to pin down but
I’ve observed the following:

* nginx not accepting connections under load
* A java app which uses netty / NIO having strange writability
semantics on channels, which confuses netty / java enough to not
properly flush written data on the socket.

I went and tested these Linux kernels:

4.20.17
4.19.32
4.14.111

And the issue(s) do not show up there.

I’m still actively chasing this up, and will report back — I haven’t
touched kernel code in 15 years so I’m a little rusty. :)

Regards,
Omar