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Date:   Wed, 11 Nov 2020 08:11:47 +0000
From:   Christoph Hellwig <hch@infradead.org>
To:     Damien Le Moal <damien.lemoal@wdc.com>
Cc:     linux-block@vger.kernel.org, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH v2 4/9] null_blk: improve zone locking
Message-ID: <20201111081147.GD23360@infradead.org>
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> +	spin_lock_init(&dev->zone_res_lock);
> +	if (dev->memory_backed)
>  		dev->zone_locks = bitmap_zalloc(dev->nr_zones, GFP_KERNEL);
> +	else
> +		dev->zone_locks = kmalloc_array(dev->nr_zones,
> +						sizeof(spinlock_t), GFP_KERNEL);
> +	if (!dev->zone_locks) {

Using the same pointer for different types is pretty horrible.  At very
least we should use a union.

But I think we'd end up with less contention if we add a new

struct nullb_zone

that contains the fields of blk_zone that we actuall care about plus:

	union {
		spinlock_t	spinlock;
		mutex_t		mutex;
	}

and then use the proper lock primitive also for the memory backed case.