linux-kernel - Re: [RFC PATCH] sched/wait: Make interruptible exclusive waitqueue wakeups reliable

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Message-ID: <CAHk-=whpfd2cmuGj4NYjJ-UoJnoGPyaTZH-dA0xEAeTvgYZFZA@mail.gmail.com>
Date:   Mon, 9 Dec 2019 10:06:59 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Felipe Balbi <balbi@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC PATCH] sched/wait: Make interruptible exclusive waitqueue
 wakeups reliable

On Mon, Dec 9, 2019 at 2:28 AM Ingo Molnar <mingo@...nel.org> wrote:
>
> Totally untested, but shows the principle: I believe interrupted
> exclusive waits should not auto-cleanup in prepare_to_wait_event().

Oleg convinced me I was wrong, and that there is no bug, so I don't
think we need anything after all.

Yes, there's a "race" between wakeup and returning the error, but
since any correct code already relies on the condition having become
true before the wakeup (and it needs to be a stable condition -
otherwise the exclusive waiters wouldn't work in the first place), the
code that returns the error will never get reached because of the

>                 if (condition)
>                         break;

part of the ___wait_event() macro.

            Linus