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, 28 Oct 2020 15:47:41 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     linux-block@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        David Runge <dave@sleepmap.de>, linux-rt-users@vger.kernel.org,
        Jens Axboe <axboe@kernel.dk>, linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Daniel Wagner <dwagner@suse.de>
Subject: Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu_done
Message-ID: <20201028144741.zvldomzk2pwj4zq6@linutronix.de>
References: <20201028065616.GA24449@infradead.org>
 <20201028141251.3608598-1-bigeasy@linutronix.de>
 <20201028141251.3608598-3-bigeasy@linutronix.de>
 <20201028144453.GA18610@infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20201028144453.GA18610@infradead.org>
Precedence: bulk

On 2020-10-28 14:44:53 [+0000], Christoph Hellwig wrote:
> On Wed, Oct 28, 2020 at 03:12:51PM +0100, Sebastian Andrzej Siewior wrote:
> >  static int blk_softirq_cpu_dead(unsigned int cpu)
> >  {
> > -	/*
> > -	 * If a CPU goes away, splice its entries to the current CPU
> > -	 * and trigger a run of the softirq
> > -	 */
> > -	local_irq_disable();
> > -	list_splice_init(&per_cpu(blk_cpu_done, cpu),
> > -			 this_cpu_ptr(&blk_cpu_done));
> > -	raise_softirq_irqoff(BLOCK_SOFTIRQ);
> > -	local_irq_enable();
> > -
> > +	blk_complete_reqs(&per_cpu(blk_cpu_done, cpu));
> >  	return 0;
> 
> How can this be preempted?  Can't we keep using this_cpu_ptr here?

cpu of the dead CPU != this CPU.

Sebastian