linux-kernel - Re: [PATCH v1 2/2] Add selftests for pidfd polling

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Date:   Thu, 25 Apr 2019 15:07:48 -0700
From:   Daniel Colascione <dancol@...gle.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>, Jann Horn <jannh@...gle.com>,
        Jann Horn <jann@...jh.net>,
        Jonathan Kowalski <bl0pbl33p@...il.com>,
        Android Kernel Team <kernel-team@...roid.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Michal Hocko <mhocko@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Serge Hallyn <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>,
        Sandeep Patil <sspatil@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Murray <timmurray@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tycho Andersen <tycho@...ho.ws>
Subject: Re: [PATCH v1 2/2] Add selftests for pidfd polling

On Thu, Apr 25, 2019 at 2:29 PM Christian Brauner <christian@...uner.io> wrote:
> This timing-based testing seems kinda odd to be honest. Can't we do
> something better than this?

Agreed. Timing-based tests have a substantial risk of becoming flaky.
We ought to be able to make these tests fully deterministic and not
subject to breakage from odd scheduling outcomes. We don't have
sleepable events for everything, granted, but sleep-waiting on a
condition with exponential backoff is fine in test code. In general,
if you start with a robust test, you can insert a sleep(100) anywhere
and not break the logic. Violating this rule always causes pain sooner
or later.

Other thoughts: IMHO, using poll(2) instead of epoll would simplify
the test code, and I think we can get away with calling
pthread_exit(3) instead of SYS_exit.