Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 11 Nov 2020 21:45:00 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Paul Bolle <pebolle@tiscali.nl>
Cc:     linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        Scott Wood <swood@redhat.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        x86@kernel.org
Subject: Re: [tip: sched/core] sched: Fix balance_callback()
Message-ID: <20201111204500.GR2628@hirez.programming.kicks-ass.net>
References: <20201023102346.203901269@infradead.org>
 <160508300397.11244.13967684821070428528.tip-bot2@tip-bot2>
 <6356963f376a0798e8c939f813c2efe05d32c6d7.camel@tiscali.nl>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <6356963f376a0798e8c939f813c2efe05d32c6d7.camel@tiscali.nl>
Precedence: bulk

On Wed, Nov 11, 2020 at 09:30:42PM +0100, Paul Bolle wrote:
> tip-bot2 for Peter Zijlstra schreef op wo 11-11-2020 om 08:23 [+0000]:
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > [...]
> > +static void do_balance_callbacks(struct rq *rq, struct callback_head *head)
> > +{
> > +	void (*func)(struct rq *rq);
> > +	struct callback_head *next;
> > +
> > +	lockdep_assert_held(&rq->lock);
> > +
> > +	while (head) {
> > +		func = (void (*)(struct rq *))head->func;
> > +		next = head->next;
> > +		head->next = NULL;
> > +		head = next;
> 
> Naive question: is there some subtle C-issue that is evaded here by setting
> head->next to NULL prior to copying over it?
> 
> (I know this piece of code only got copied around in this patch and this is
> therefor not something that this patch actually introduced.)

It's like list_del_init(), it zeros the entry before unlinking it.
queue_balance_callback() relies on this.