linux-kernel - Re: [PATCH 1/2] sched/swait: allow swake

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Date:   Thu, 9 Nov 2017 11:23:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Kr??m???? <rkrcmar@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/swait: allow swake_up() to return

On Thu, Nov 09, 2017 at 05:18:53PM +0800, Peter Xu wrote:
> Let swake_up() to return whether any of the waiters is waked up. One use
> case of it would be:
> 
>   if (swait_active(wq)) {
>     swake_up(wq);
>     // do something when waiter is waked up
>     waked_up++;
>   }

The word is 'woken', and no that doesn't work. All it says is that there
was a waiter, not that you were to one to wake it. Another concurrent
wakeup might have done so.

> 
> Logically it's possible that when reaching swake_up() the wait queue is
> not active any more, and here doing something like waked_up++ would be
> inaccurate.  To correct it, we need an atomic version of it.
> 
> With this patch, we can simply re-write it into:
> 
>   if (swake_up(wq)) {
>     // do something when waiter is waked up
>     waked_up++;
>   }
> 
> After all we are checking swait_active() inside swake_up() too.

We're not in fact; you've been staring at old code; see commit:

  35a2897c2a30 ("sched/wait: Remove the lockless swait_active() check in swake_up*()")


Also, you're changing the interface relative to the regular wait
interface. The two should be similar wherever possible.