From: Peter Zijlstra <peterz@infradead.org>
To: Peter Xu <peterx@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
Radim Kr??m???? <rkrcmar@redhat.com>,
Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] sched/swait: allow swake_up() to return
Date: Thu, 9 Nov 2017 11:23:03 +0100 [thread overview]
Message-ID: <20171109102303.vsetymxc6rllomhh@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20171109091854.24367-2-peterx@redhat.com>
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.
next prev parent reply other threads:[~2017-11-09 10:23 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20171109091854.24367-1-peterx@redhat.com>
2017-11-09 9:18 ` [PATCH 1/2] sched/swait: allow swake_up() to return Peter Xu
2017-11-09 10:06 ` Paolo Bonzini
2017-11-10 7:12 ` Peter Xu
2017-11-09 10:23 ` Peter Zijlstra [this message]
2017-11-10 7:10 ` Peter Xu
2017-11-10 8:05 ` Peter Zijlstra
2017-11-13 3:33 ` Peter Xu
2017-11-13 5:19 ` Peter Xu
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