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Date:   Tue, 23 Nov 2021 10:59:53 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Christoph Hellwig <hch@infradead.org>, linux-block@vger.kernel.org
Subject: Re: [PATCH 3/3] block: only allocate poll_stats if there's a user of
 them
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On Tue, Nov 23, 2021 at 11:58:42AM -0700, Jens Axboe wrote:
> On 11/23/21 11:50 AM, Christoph Hellwig wrote:
> >> +	spin_lock_irq(&q->stats->lock);
> >> +	if (q->poll_stat) {
> >> +		spin_unlock_irq(&q->stats->lock);
> >> +		kfree(poll_stat);
> >> +		return true;
> >> +	}
> >> +	q->poll_stat = poll_stat;
> >> +	spin_unlock_irq(&q->stats->lock);
> > 
> > If we'd use a cmpxchg to install the pointer we could keep the
> > blk_queue_stats definition private.
> 
> How about we just move this alloc+enable logic into blk-stat.c instead?

That's a good idea either way.  But I think cmpxchg is much better
for installing a pointer than an unrelated lock.