linux-kernel - Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu

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Date:   Wed, 28 Oct 2020 15:47:41 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-block@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        David Runge <dave@...epmap.de>, linux-rt-users@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Wagner <dwagner@...e.de>
Subject: Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu_done

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